Preserve empty JSON schema object maps

[chubes4]

Summary

• Adds a JSON-serializable parameter accessor for FunctionDeclaration that preserves empty JSON Schema object-map fields as {}.
• Updates FunctionDeclaration::jsonSerialize() to use the safe parameter representation.
• Updates the shared OpenAI-compatible tool payload builder to use the safe parameter representation when embedding function declarations.

Fixes #233.

Testing

• vendor/bin/phpunit tests/unit/Tools/DTO/FunctionDeclarationTest.php --filter testJsonSerializationPreservesEmptySchemaObjectMaps
• vendor/bin/phpunit tests/unit/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModelTest.php --filter testPrepareToolsParamPreservesEmptySchemaObjectMaps
• vendor/bin/phpunit tests/unit/Tools/DTO/FunctionDeclarationTest.php
• vendor/bin/phpunit tests/unit/Providers/OpenAiCompatibleImplementation/AbstractOpenAiCompatibleTextGenerationModelTest.php
• composer phpcs
• composer phpstan

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the focused serialization fix and tests from the issue reproduction; Chris remains responsible for review and merge.

[github-actions]

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Co-authored-by: chubes4 <[email protected]>
Co-authored-by: felixarntz <[email protected]>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (6317042) to head (db2e83e).
⚠️ Report is 7 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #234      +/-   ##
============================================
+ Coverage     88.12%   88.22%   +0.09%     
- Complexity     1213     1226      +13     
============================================
  Files            60       61       +1     
  Lines          3934     3967      +33     
============================================
+ Hits           3467     3500      +33     
  Misses          467      467              

Flag	Coverage Δ
unit	88.22% <100.00%> (+0.09%)	⬆️

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[felixarntz]

@chubes4 Thank you for the PR! Mostly looks good, a few minor things to address.

[chubes4]

Thank you! Will fix everything noted.

[chubes4]

Thanks for the review, @felixarntz. I addressed the feedback in db2e83e: prepareToolsParam() now uses jsonSerialize() directly, the public helper was removed, and the new @since annotations use n.e.x.t.