This update moves mail and live chat related tests to run after installation, allowing automated builds to run them in parallel and complete faster. It also improves test coverage by checking the full installed application setup and tidies test file organization for easier maintenance.

\* = crm_livechat, im_livechat, test_mail, website_livechat

The opportunity is taken to clean up and standardize headers/imports.

- clean up obsolete # at the top of files
- regroup imports from `odoo.tests` all together, apply import guidelines, remove unused imports
- always have tagged usage with double quote, and in this order: test specific tags, post_install, -at_install

Performance
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Post-install tests are parallelized on runbot, allowing to reduce build times.

Before/after

mail: 18.81s -> 0s
test_mail: 113.7s -> 2.53s

With this PR 130s can be gained on each runbot build with splits.

Coverage
--------
Post-install tests cover the full application (with all overrides installed) rather than a particular sub-set of the code.

Tests of sub-set of the code are desirable too, but they are already tested with single-app builds on runbot.

Maintenace
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Post-install tests are easier to maintain and to run, as dropping the database and installing it again is not necessary to run them.

Sales orders now suggest adding the customer as a follower when a message is sent in the chatter, if they are not already following. This helps keep customers included in relevant communication and reduces the chance of missed updates.

Suggest the customer as a follower (if not already the case) when sending a message in the chatter.

task-3246613

The customer portal down payment choice now looks and behaves more like a standard switch. This makes the payment selection clearer for customers and improves the checkout experience without changing the underlying payment process.

The downpayment option on portal doesn't look like a switch.
this commit transform the buttons into the default btn-group component
to act like a regular switch.

However since the page needs to refresh, we have to apply the classes
manually with a switch triggering the checked attribute to obtain the expected visual result.

task-3499055

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This update aligns enterprise e-commerce rental and tax-related checkout components with recent core platform changes. It keeps delivery returns, cart updates, and external tax integrations using the right underlying behavior, reducing maintenance risk and helping checkout flows remain reliable.

* Additional cleanup/improvements

Community PR: https://github.com/odoo/odoo/pull/131184