Prevent log_event metadata keyword mismatches from masking HTTP responses

[paullizer]

Issue

Application code can pass unsupported structured-metadata keywords to functions_appinsights.log_event(). In the conversation-fork conflict path, custom_dimensions= raises TypeError, replacing the intended HTTP 409 eligibility conflict with HTTP 500. Additional fork cleanup paths use the similarly unsupported properties= keyword.

Steps to Reproduce

1. Submit a personal-conversation fork request that correctly raises ConversationForkConflictError.
2. Let the route attempt to log the conflict with custom_dimensions=.
3. Observe that the logger call raises before the route can return its intended response.

Expected Behavior

The conflict is logged with canonical structured metadata and the route returns HTTP 409. All application log_event() callers use only supported arguments, with automated coverage preventing future contract drift.

Actual Behavior

The conflict logger raises TypeError: log_event() got an unexpected keyword argument 'custom_dimensions', converting the intended HTTP 409 into HTTP 500. Five cleanup/fallback calls also use unsupported properties= metadata.

Impact

Users receive an incorrect server-error response for a valid eligibility conflict, diagnostics are disrupted, and the same failure class can occur in other exception-handling paths.

Acceptance Criteria

• Replace all unsupported custom_dimensions= and properties= arguments in application log_event() calls with canonical extra= metadata.
• Add an app-wide static contract test for log_event() call signatures under application/.
• Add a route regression proving a fork conflict remains HTTP 409 while structured metadata is logged.
• Preserve existing logging levels, messages, traceback behavior, and metadata values.

Notes

Current findings are in route_backend_conversations.py and functions_simplechat_operations.py. No duplicate issue was found in the initial repository search. Roadmap triage: priority P1, size M.

[paullizer]

Implemented on branch paullizer-audit-log-events for version 0.250.101.

• Standardized all nine incompatible conversation-fork log_event() metadata calls on extra= (four custom_dimensions= and five properties= call sites).
• Added an AST-based contract test covering Python callers throughout application/.
• Added a route regression proving ConversationForkConflictError logs structured metadata and still returns HTTP 409.
• Added fix documentation and release notes.
• Validation completed: route-policy checks, 15 targeted logging/fork/privacy tests, Python compilation, and whitespace checks passed.

No unresolved blockers or follow-ups were found.

[paullizer]

Shipped in v0.250.101 via #1116.

Release note entries:

• v0.250.101: Application-Wide Log Event Contract Guard

Full notes: https://microsoft.github.io/simplechat/explanation/release_notes/

Closing as completed. This issue stayed open after the fix merged because pull requests target Development while the repository default branch is main, so GitHub never applied the Fixes # keyword automatically.