Problem

When beta.chat.completions.parse() receives malformed or truncated JSON from the API (with finish_reason: "stop"), a raw pydantic.ValidationError is raised directly to the user with no context about what went wrong or what the original content was.

This is confusing because:

• The error doesn't indicate it came from parsing the API response
• The raw content that failed to parse is lost
• Users can't distinguish between a bug in their code vs bad API output

Solution

Added a new ContentFormatError exception (extends OpenAIError, following the pattern of LengthFinishReasonError) that wraps both pydantic.ValidationError and json.JSONDecodeError in _parse_content().

The new error:

• Provides a clear message explaining the response didn't match the expected format
• Preserves the raw content string via raw_content attribute for debugging
• Chains the original exception as __cause__ so the full traceback is available

Changes

• src/openai/_exceptions.py: Added ContentFormatError class
• src/openai/__init__.py: Exported ContentFormatError
• src/openai/lib/_parsing/_completions.py: Wrapped _parse_content() with try/except
• tests/lib/chat/test_completions.py: Added tests for malformed JSON and schema mismatch

Example

try:
    completion = client.beta.chat.completions.parse(
        model="gpt-4o",
        messages=[...],
        response_format=MyModel,
    )
except openai.ContentFormatError as e:
    print(e.raw_content)  # The raw JSON string that failed to parse

Fixes #1763