[IMP] purchase_deposit: make the deposit over total check optional

[mav-adhoc]

What

Forward port of #3172, as asked for there.

The "deposit can't be greater than the PO total" check added in #2788 never
reached this branch, so it arrives here already behind the company setting
Limit Deposits to Order Total, disabled by default. Nothing changes for an
existing deployment: a deposit over the order total is still accepted, and the
ceiling is opt-in from Purchase > Configuration > Settings.

Why it is disabled by default

1. The two sides of the comparison are not the same base. It sums the deposit
lines' price_unit, which is tax excluded, and compares it against
order.amount_total, which is tax included. The effective ceiling therefore depends on
whether the order carries taxes: the untaxed total on a tax exempt order, the tax
included total on a taxed one. Deposit lines also carry product_qty = 0, so they never
contribute to amount_total themselves.

2. On a multi-currency order the sum it compares has no single right value. When the
order is in a foreign currency and the vendor bills the deposits in the company
currency, each deposit line holds an amount converted from the bill. Whichever rate that
conversion uses, the sum cannot serve as a hard limit:

• at each bill's own date (what the module does, see [FIX] purchase_deposit: convert deposit amounts to the order currency #3142) the amount left to bill on
  the order floats with the exchange rate. An order advanced in fixed instalments of the
  company currency stops adding up, and the last instalment is refused even though every
  single amount is right.
• at the order's own rate (purchase.order.currency_rate) a fully advanced order does
  add up — but only while that rate is still meaningful. On an order paid over a year or
  more in a high-inflation company currency, the order rate is stale and it inflates
  every later deposit instead.

That is not a bug in either conversion — an exchange difference on a prepayment is real
and has to be recognised somewhere. But it has to be recognised in the accounting of the
bills, not charged against a commercial control that then refuses the operation. For
reference, core sale handles the same situation with a non-blocking warning in the
wizard (display_invoice_amount_warning) rather than an error.

There is no migration script: the setting starts disabled everywhere.

The error message states the deposit total, the order total and the currency, so a
refusal can be read without opening the order.

The readme fragment is renamed CONFIGURATION.rst → CONFIGURE.md, the name the readme
generator looks for; the old one never reached README.rst. README.rst itself is left
to the bot, as the CI expects for PRs that touch readme fragments.

Test plan

• test_not_allow_deposit_greater_than_po_total and
  test_not_allow_deposit_over_total_with_deposit_already_invoiced cover the check with
  the setting enabled. Both fail if the guarded block is removed.
• test_allow_deposit_greater_than_po_total_by_default covers the default: the same
  deposit that the check refuses is created.
• Full suite: 0 failed, 0 error(s) of 9 tests.

[mav-adhoc]

@HviorForgeFlow Can you review it please? Thanks!

[UsmanGhias]

Clean patch for OCA/purchase-workflow.

Observations:

• Code structure follows standard OCA guidelines and cleanly isolates the extension logic.

Ready for testing on standard environments.

Best,
Usman

[HviorForgeFlow]

/ocabot merge minor

[OCA-git-bot]

Hey, thanks for contributing! Proceeding to merge this for you.
Prepared branch 19.0-ocabot-merge-pr-3182-by-HviorForgeFlow-bump-minor, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at c3edd5c. Thanks a lot for contributing to OCA. ❤️