Fix #985: Handle PydanticInvalidForJsonSchema for unserializable BaseModel

[hulincup]

Description

Fixes #985

When a BaseModel contains fields that cannot be serialized to JSON schema (e.g., Callable fields), the to_strict_json_schema() function raises PydanticInvalidForJsonSchema. Currently, this exception propagates up and breaks the chat call.

The issue description notes that RowTypeInfo schemas gracefully fall back to prompt engineering when the connection can't translate them natively, but BaseModel schemas that can't be serialized raise an exception instead. This inconsistency should be fixed.

Changes

This PR catches PydanticInvalidForJsonSchema in the three connection types that use to_strict_json_schema() or transform_schema():

1. OpenAI (openai_chat_model.py):

   • Catches exception in _native_response_format()
   • Falls back to prompt engineering (returns None)
   • Logs a warning message
   • Added logger import

2. Azure OpenAI (azure_openai_chat_model.py):

   • Catches exception in _native_response_format()
   • Falls back to prompt engineering (returns None)
   • Logs a warning message
   • Updated docstring to document the fallback behavior

3. Anthropic (anthropic_chat_model.py):

   • Catches exception in _native_output_config()
   • Falls back to prompt engineering (returns None)
   • Logs a warning message
   • Updated docstring to document the fallback behavior

Testing

Added comprehensive test file test_unserializable_output_schema.py with three test cases:

• test_openai_handles_unserializable_schema(): Verifies OpenAI falls back to prompt when schema cannot be serialized
• test_azure_handles_unserializable_schema(): Verifies Azure falls back to prompt when schema cannot be serialized
• test_anthropic_handles_unserializable_schema(): Verifies Anthropic falls back to prompt when schema cannot be serialized

Each test uses a BadModel with a Callable field that cannot be converted to JSON schema, and verifies that:

1. No exception is raised
2. The native structured output format is not included in the request (fallback to prompt engineering)

Verification

The fix ensures that:

• Connections handle unserializable BaseModel schemas gracefully
• Behavior is consistent with RowTypeInfo fallback
• Users receive a warning log when fallback occurs
• Existing functionality for serializable schemas is unchanged

Related Issues

• Fixes [Bug] An output schema pydantic cannot render raises instead of taking the prompt fallback #985