Before this commit _compute_parent_line_id make the assumption that a parent line can only be linked to once child: When the parent_line_id is set, it's remove from the pool of possible parent_line_id

Actually this is not the case, different upsell order can have a line that have the same parent.

Upsell order A can have line 1 with the same parent as the line 2 of Upsell order B.

Before this commit if the parent line of both order were computed at the same time one of them would end up without parent.

This situation is getting worse since odoo@42008de (saas-18.1) were all the parent_line_id are recomputed on every upsell order (even already confirmed one) as soon as line on the parent order are touched

The follow use case lead to duplicate line on the parent

A parent subscription A with 1 product Z
Create an upsell B with a line that upsell 1 product Z Create an upsell C with a line that upsell 1 product Z On both upsell the parent line is set properly

Confirm Upsell B, the parent line on Upsell C is set to false Confirm Upsell B, the parent subscription ends up with a two line for the product Z

After this commit,
parent_line_id are computed per order, so one order cannot have two lines with the same parent_line_id but two (or more) different order can have each one line with the same parent_line_id even if their parent_line_id are computed at the same time.

Forward-Port-Of: odoo/enterprise#84340
Forward-Port-Of: odoo/enterprise#84255

**PURPOSE**

ESG (Environmental, Social, and Governance) module was built with one clear mission: to make ESG reporting simple, integrated, and affordable for every business.
Today, ESG is no longer a nice-to-have, it's a legal, financial, and reputational necessity. With the EU's Corporate Sustainability Reporting Directive (CSRD) now in force, companies are required to report their environmental, social, and governance performance using formal frameworks like the European Sustainability Reporting Standards (ESRS).
But beyond compliance, the pressure is mounting from all sides: investors are assessing ESG risks before funding businesses, major clients are adding sustainability criteria to supplier evaluations, and consumers are choosing brands that act responsibly. At the same time, ESG opens up massive business opportunities; from lowering energy costs and improving employee retention to building long-term trust and brand loyalty.
That's where Odoo comes in. By natively integrating with apps like Accounting, Fleet, Payroll, and Employees, we automate ESG data collection, eliminating the manual burden and reducing the cost and time spent collecting the data required for carbon footprint reporting. Each business activity can be tracked, through expense type of account move lines, in real time, giving you an up-to-date view of your emissions and enabling you to measure the immediate impact of your sustainability efforts.
If you work with a carbon consultant, their configuration becomes a reusable asset, making year-on-year updates seamless. For environmental consultancies, we also offers the most affordable carbon tracking tool on the market enabling you to focus on expertise services while giving clients a powerful, integrated platform without the need for expensive external tools. With secure access rights, ESG professionals can collaborate directly on client data, just like accountants do.
With Odoo, ESG impact is no longer a burden or a side project, it becomes a natural, data-driven part of how you run your business.

**MODELS**

* Source Database (`esg.database`): connect to certified emission factor databases, including regional and sector-specific sources (e.g. ADEME, IPCC). Download the DB that the system proposes and that fit your business needs or upload yours by creating a new one.
* Gaz (`esg.gas`): includes the six gases defined under the Kyoto Protocol: CO₂, CH₄, N₂O, HFCs, PFCs, and SF₆. Each gas is assigned a Global Warming Potential (GWP) value, allowing for conversion into a standard unit — CO₂-equivalents (kgCO₂e). This ensures consistency and comparability across different emission sources, aligning with GHG Protocol standards.
* Emission Sources (`esg.emission.source`): grouped into three scopes based on their origin:
  - Scope 1: Direct emissions from owned or controlled sources (e.g. fuel used in company cars or on-site combustion).
  - Scope 2: Indirect emissions from purchased energy (e.g., electricity, steam, heating).
  - Scope 3: All other indirect emissions from the value chain, including both upstream (e.g. supplier emissions, travel) and downstream (e.g. product use, waste). Scope 3 often represents the majority of total emissions.
* Emission factors (`esg.emission.factor`): coefficients which allows to convert activity data into GHG emissions.
  - Computation method: physical vs monetary (choose how emissions are calculated)
    - Physical: based on quantity and unit of measure: e.g. kg CO₂ per liter of fuel or per kWh — more precise.
    - Monetary: based on amount and currency: e.g. kg CO₂ per € spent — useful when no physical data is available.
* Assignation Rules (`esg.assignation.line`): define how the emission factors will be assigned on the account move lines.
  - Product level (e.g. laptop, printer)
  - Partner level (e.g. emissions tied to a specific supplier)
  - Account level (e.g. expense account 6700 for Office Equipment)
  => Product assignation takes precedence over partner and account, and partner assignation takes precedence over account.
* Activity type mapping (`esg.activity.type`): link each emission to a lifecycle activity type (e.g. Raw Materials, Transport, Energy).
* Other Emissions (`esg.other.emission`): emissions that are not linked to a journal entry. This allows for the inclusion of emissions from activities that are not directly tied to financial transactions, such as employee commuting or waste disposal.

**MENU ITEMS**

* Collect:
  - Emissions: automatic data synchronization. Pulls data from Accounting for purchase and expense data and provides a “New” option to add Other Emissions (not linked to a journal entry) manually. The system will assign Emission Factor if there's a pre-configured assignation rule and if not, the user can add an emission factor on an emission manually.
  - Employee commuting pivot table: uses data from the Employee (distance from home to office), Fleet (Vehicle model and associated emission factor (gCO₂/km)). 
 Apply the formula: `Days * Distance * 2 (round trip) * 3/7 (average number of office days per week) * Vehicle emission factor`

* Measure -> Environment:
  - Carbon Analytics: visualize and analyze emissions per year, scopes (1,2,3), activity types (energy, transport, etc.), etc. To highlight where to act to have the biggest impact on your company's carbon footprint.
  - Carbon Footprint Report: automatically generate your company's carbon footprint report: in tCO₂e (tonnes of CO₂ equivalent), by scopes, with gas repartition, aligned with “Le Bilan Carbone” and “GHG Protocol” requirements, available in downloadable PDF and Excel formats for submission or internal use.

* Measure -> Social:
  - Gender Parity: view gender breakdown across departments, leadership levels, locations, job positions, etc.
  - Pay Gap Calculation: automatically calculates the median hourly pay gap between male and female employees, following the EU method: `(Median male pay - Median female pay) / Median male pay * 100`

* Act:
  - Initiatives as projects: Use the Project module to manage ESG actions.
  
[ task-4517841](https://www.odoo.com/odoo/project.task/4517841)

Related to https://github.com/odoo/odoo/pull/195951

Forward-Port-Of: odoo/enterprise#79004