Steps to reproduce:
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1. Install `sale_timesheet` module
2. Create a product with:
    * Type: Service
    * Invoicing Policy: Prepaid/Fixed Price
    * Create on Order: Project & Task
    * Upsell Threshold: Set some value (e,g. 600%)
3. Create and confirm the sale order with that product
4. Create and confirm an invoice for the sale order
5. From Recorder Hours Smart button:
    * Add two timesheets for hours 3 & 4 (Combined h > Threshold Percentage/100)

Observation:
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* When adding a single 7h timesheet, an upsell warning activity is correctly created for the salesperson in the chatter.
* When adding multiple incremental timesheets whose combined duration exceeds the threshold, no upsell warning activity is created.

Issue:
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* The `_compute_field_value` method filters on `invoice_status != 'upselling'` before recomputing the new invoice status.
* After the first incremental timesheet is added, the sales order already has the upselling status from the previous computation.
* As a result, subsequent computations skip the upsell activity logic, even though `has_displayed_warning_upsell` is still `False` and no warning activity has yet been created.

Solution:
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* Removes the `so.invoice_status != 'upselling'` condition from the filter.
* The order-level `invoice_status` was never the right place to control upsell activity creation. An order can have `invoice_status == 'upselling'` for various reasons (delivered > invoiced), but that doesn't mean all lines have already triggered their upsell warnings.

opw-6117754