Fix multiple OpenAI-compatible function responses

[zahidgh]

Summary

• serialize multiple function responses from one message as separate tool messages
• preserve the existing single-function-response payload shape
• add unit coverage for multiple tool responses returned by a single execution pass

When a model requests multiple tools in one response, the resulting function responses can be grouped in one user message. The OpenAI-compatible adapter previously handled only a single function response and attempted to process grouped responses as regular content. This change emits one API tool message per response, retaining each tool_call_id.

Fixes #286

Validation

• composer test -- --filter AbstractOpenAiCompatibleTextGenerationModelTest — 66 tests, 165 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: stefanolissa <satollo@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.59%. Comparing base (a164848) to head (44d0cba).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #290      +/-   ##
============================================
+ Coverage     86.58%   86.59%   +0.01%     
- Complexity     1383     1385       +2     
============================================
  Files            69       69              
  Lines          4449     4454       +5     
============================================
+ Hits           3852     3857       +5     
  Misses          597      597              

Flag	Coverage Δ
unit	86.59% <100.00%> (+0.01%)	⬆️

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