feat: refactor to multi-provider abstraction with registry pattern

[dkargatzis]

• Move providers from src/core/ai_providers to src/providers
• Consolidate integration logic into provider classes
• Add OpenAI, Bedrock, and Vertex AI provider implementations
• Update all agents to use new get_chat_model() factory
• Fix structured output schema for LLMEvaluationResponse
• Update tests to mock get_chat_model instead of ChatOpenAI

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 21.01167% with 203 lines in your changes missing coverage. Please review.

❌ Your patch status has failed because the patch coverage (21.0%) is below the target coverage (80.0%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (24.2%) is below the target coverage (80.0%). You can increase the head coverage or adjust the target coverage.
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@@           Coverage Diff           @@
##            main     #20     +/-   ##
=======================================
- Coverage   25.4%   24.2%   -1.3%     
=======================================
  Files         59      67      +8     
  Lines       4098    4632    +534     
=======================================
+ Hits        1044    1124     +80     
- Misses      3054    3508    +454     

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