Replace payum's plain-PHP GetHttpRequest action with the header-aware one by default

[loevgaard]

Implements A3 of #71 (marked optional there — this is the proposal; small and self-contained, easy to drop if you'd rather keep it explicit).

NotifyAction reads QuickPay-Checksum-Sha256 off GetHttpRequest::$headers, and among payum/core's own bridges only the Symfony one sets it. Payum's core config puts its plain-PHP GetHttpRequestAction in place before populateConfig() runs, so a plain-PHP Payum that did not know to swap in HeaderAwareGetHttpRequestAction (#64) rejected every callback as unsigned, 400, silently, for every payment. The README said so; nothing enforced it.

QuickpayGatewayFactory::populateConfig() now replaces payum.action.get_http_request with HeaderAwareGetHttpRequestAction when — and only when — the configured value is payum's exact plain-PHP class:

• PayumBundle's Symfony bridge (a different class) is left alone → Sylius/Symfony unaffected;
• anything a consumer configured deliberately — a subclass of payum's action, another action, the header-aware one already — is left alone (tested per case);
• what is swapped is the one value nobody chose.

A real-gateway test executes GetHttpRequest through the factory-built gateway with $_SERVER['HTTP_QUICKPAY_CHECKSUM_SHA256'] set and reads the header back. README (Callbacks), docs/UPGRADE-2.0.md and CLAUDE.md updated; the e2e harness keeps its explicit registration as the reference for consumers wiring their own (comment says it is now redundant).

composer all and the dependency analyser green (322 tests).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.43%. Comparing base (ef3db7f) to head (cc335de).
⚠️ Report is 2 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x      #78      +/-   ##
============================================
+ Coverage     98.41%   98.43%   +0.01%     
- Complexity      202      206       +4     
============================================
  Files            19       19              
  Lines           506      512       +6     
============================================
+ Hits            498      504       +6     
  Misses            8        8              

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[loevgaard]

Rebased onto 2.x (README and a test import conflicted with #72/#76); no functional change. Merging once CI is green.