Add provider hook for preparing function declarations

[zahidgh]

Summary

• add a protected no-op hook for preparing function declarations before serialization
• invoke the hook from the shared OpenAI-compatible tools path
• add unit coverage proving a provider override can adapt a declaration

Provider APIs accept different subsets of JSON Schema for tool input parameters. This hook lets a provider normalize its own declarations without changing the canonical FunctionDeclaration DTO or affecting the default payload.

Fixes #256

Validation

• composer test:unit — 1,165 tests, 4,262 assertions
• composer lint — PHPCS and PHPStan pass
• git diff --check

AI assistance disclosure

AI assistance was used for drafting and test scaffolding. The implementation and validation were reviewed before submission.

[github-actions]

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Co-authored-by: zahidgh <zahidui@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.50%. Comparing base (a31b0ec) to head (5a1247a).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #280   +/-   ##
=========================================
  Coverage     86.49%   86.50%           
- Complexity     1327     1328    +1     
=========================================
  Files            68       68           
  Lines          4295     4298    +3     
=========================================
+ Hits           3715     3718    +3     
  Misses          580      580           

Flag	Coverage Δ
unit	86.50% <100.00%> (+<0.01%)	⬆️

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