Add provider-supplied request authentication

[chubes4]

Summary

• Add an optional provider contract for supplying a custom RequestAuthenticationInterface instance.
• Teach ProviderRegistry to use provider-supplied authentication before falling back to metadata/env-derived authentication.
• Add focused registry tests covering custom auth storage and model dependency binding.

Fixes #237.

Testing

• php -l src/Providers/Contracts/ProviderWithRequestAuthenticationInterface.php && php -l src/Providers/ProviderRegistry.php && php -l tests/mocks/MockCustomRequestAuthentication.php && php -l tests/mocks/MockCustomAuthProvider.php && php -l tests/mocks/MockCustomAuthModel.php
• vendor/bin/phpunit tests/unit/Providers/ProviderRegistryTest.php --filter 'testRegisterProvider(Uses|Binds)ProviderSuppliedRequestAuthentication'
• composer phpcs && composer phpstan
• composer test ran all 1090 tests/assertions, but this local PHP 8.5 runtime reports 83 existing reflection tests as risky because ReflectionMethod::setAccessible() / ReflectionProperty::setAccessible() now emit deprecations.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the minimal provider-auth interface, registry wiring, focused tests, and local verification. Chris remains responsible for review and acceptance.

[github-actions]

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Co-authored-by: chubes4 <extrachill@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.13%. Comparing base (99a65e8) to head (3624d24).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #238   +/-   ##
=========================================
  Coverage     88.12%   88.13%           
- Complexity     1213     1215    +2     
=========================================
  Files            60       60           
  Lines          3934     3937    +3     
=========================================
+ Hits           3467     3470    +3     
  Misses          467      467           

Flag	Coverage Δ
unit	88.13% <100.00%> (+<0.01%)	⬆️

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