feat(api): improve JSON error logging

[steebchen]

Summary

• Enhances error logging for JSON parsing failures in Anthropic and Chat API endpoints
• Adds detailed contextual information to logs including request ID, headers, and error details
• Improves observability and debugging of invalid JSON request bodies and provider error responses

Changes

Anthropic API

• Added structured logging for JSON parsing errors in /v1/messages endpoint
• Logs include request ID, error message, content type, user agent, and referer headers
• Throws HTTP 400 with detailed error message on invalid JSON

Chat API

• Added detailed error logging for JSON parsing failures in chat completions endpoint
• Logs include request ID, error message, content type, user agent, and referer headers
• Enhanced logging for failures to parse provider error responses at multiple stages
• Logs include provider, model, HTTP status, status text, parse error details, and response preview
• Returns structured error responses with detailed messages on invalid JSON

Test plan

• Verified JSON parsing errors trigger detailed logs with contextual information
• Confirmed HTTP 400 responses are returned with appropriate error messages on invalid JSON
• Tested error response parsing failures are logged with relevant metadata
• Ensured no regressions in normal request handling and error propagation

🌿 Generated by Terry

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ℹ️ Tag @terragon-labs to ask questions and address PR feedback

📎 Task: https://www.terragonlabs.com/task/ffe4f3be-2011-4a6b-8915-f717b5afcf07

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✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch terragon/improve-json-error-logging

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