Extract duplicated session ID extraction + validation logic from
SessionAuthMiddleware::call() and require_session() into two shared
helper functions: extract_session_id and validate_and_build_identity.
Removes ~60 lines of duplicated code while preserving behavior:
- Both call sites now rely on the same extraction/validation path
- User-Agent default remains empty string (existing behavior unchanged)
- Error response format (401 with header/validation messages) unchanged
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
