fix: add chat session load error diagnostics

[simple-agent-manager]

Summary

• Wraps getSession() and getMessages() calls in the chat session detail route with try/catch blocks
• Logs full error details (name, message, stack) via structured logging
• Persists errors to the observability database (platform_errors table) for /admin/errors visibility
• Returns diagnostic details (stack trace, error name/message) to admin users while showing generic error to regular users
• Includes a requestId for correlation across logs
• Adds context column to persistError for searchable structured metadata
• Adds deployment troubleshooting docs for chat session errors

Context

The "internal server error" when loading chat sessions has been recurring but the root cause is unknown because the error was being swallowed without logging. This PR adds the instrumentation needed to capture the actual error the next time it occurs.

Cherry-picked from the investigating agent's branch sam/keep-running-errors-where-01kqxn (task 01KQXN5Q8TWAGRZJ5YVRR29FH2), cleaned up to remove unrelated auto-delete TTL changes that were already merged in PR #916.

Test plan

• Typecheck passes
• 105 related unit tests pass (chat-session-agent-routing, observability, row-schemas)
• CI green
• Deploy to production via merge to main

Agent Preflight

• Change class: cross-component-change
• Impact: API error handling only — no behavior change for successful requests
• Constitution: no hardcoded values, requestId generated dynamically

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.7% Duplication on New Code

See analysis details on SonarQube Cloud