Summary

Complete the RFC 9457 implementation by adding application/problem+json content type, type URI documentation links, content negotiation, and the remaining standard fields.

Phase 1 (#419, implemented in PR #457) added the core error metadata (ErrorCategory, ErrorCode, ErrorDetail, retryable, retry_after, instance). Phase 2 makes the API fully RFC 9457 compliant.

Design (from #419 Phase 2)

• application/problem+json content type on error responses
• type URI field linking to error documentation (e.g. https://synthorg.io/docs/errors/validation)
• Content negotiation middleware via Accept header — clients that send Accept: application/problem+json get RFC 9457 format; others get the current ApiResponse envelope
• title field — human-readable summary of the error type (static per category)
• detail field — occurrence-specific human-readable explanation

Scope

• Add title and detail fields to ErrorDetail
• Add type URI field (points to docs error reference)
• Content negotiation: Accept: application/problem+json returns RFC 9457 format directly (not wrapped in ApiResponse)
• Default behavior unchanged — ApiResponse envelope with error_detail field (backward compatible)
• Error documentation page at docs/errors/ listing all error codes and categories
• Update OpenAPI schema to document both response formats

Dependencies

• feat: implement RFC 9457 structured error responses for API #419 Phase 1 (done, PR feat: add RFC 9457 structured error responses (Phase 1) #457)

Acceptance Criteria

• Error responses include type, title, detail fields
• Accept: application/problem+json returns RFC 9457 format directly
• Default response format unchanged (backward compatible)
• Error documentation page generated/written
• type URIs resolve to documentation
• OpenAPI schema documents both response formats
• Content negotiation tested with both Accept header values