Fix application-wide log event metadata contract

[Paul Lizer (paullizer)]

Summary

• standardize nine incompatible conversation-fork log_event() metadata calls on extra=
• add an AST contract test covering Python callers throughout application/
• preserve the fork eligibility conflict response as HTTP 409 with route-level regression coverage
• bump SimpleChat to 0.250.101 and add fix documentation and release notes

Validation

• route blueprint policy inventory, unauthenticated policy contract, and policy coverage checks
• 15 targeted conversation-fork, logging-contract, and privacy/log-sanitization tests
• changed-file Python compilation and whitespace checks

Closes #1112