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 <extrachill@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.21%. Comparing base (6317042) to head (824774d).

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #234      +/-   ##
============================================
+ Coverage     88.12%   88.21%   +0.08%     
- Complexity     1213     1225      +12     
============================================
  Files            60       60              
  Lines          3934     3963      +29     
============================================
+ Hits           3467     3496      +29     
  Misses          467      467              

Flag	Coverage Δ
unit	88.21% <100.00%> (+0.08%)	⬆️

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