Financial - Make Payment::create idempotent for a repeated trxn_id

[mattwire]

Overview

A payment processor webhook racing a synchronous front-end/back-office confirmation of the same charge (e.g. paying an existing pending contribution via CRM_Contribute_Form_Contribution_Confirm) can both read the contribution as not-yet-completed and both go on to record a payment for it. Add a per-contribution lock via Civi::lockManager() around create(), and treat a second payment carrying an already-recorded trxn_id as a no-op success (returning the existing FinancialTrxn) rather than creating a duplicate. Every caller gets this for free with no changes needed at the call site. Includes a regression test using APIv4 Payment::create/get.

Alternative to the reject-with-exception approach in fix-payment-create-race-lock - see that branch for comparison.

Before

Two payments recorded (or crash for one of them) if user-side and webhook record payment at the same time.

After

One payment recorded and both sides think they recorded it - desired outcome.

Technical Details

Adds a lock and relies on trxn_id being unique (which is enforced by DB entity).
The downstream code has 15 seconds to complete otherwise it will throw a CRM_Core_Exception. If downstream code completes in time the payment record is returned just the same as for the first caller.

Comments

This should be a cleaner solution than #36401 because it doesn't require code changes anywhere else.

[civibot]

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