MCP Track B Phase B4: add guarded personal chat write tool

[Bionic711]

Summary

Add send_personal_chat_message to the inbound SimpleChat MCP server only after read-only personal tools, auth, governance, telemetry, and tests are stable.

Scope

• Add send_personal_chat_message as a personal-scope-only inbound MCP tool.
• Require a valid delegated user identity.
• Validate conversation ownership/access when conversation_id is supplied.
• Define behavior for new-conversation creation.
• Preserve content safety behavior.
• Preserve model invocation and token/cost logging.
• Add rate limits and abuse protections.
• Bound response size.
• Add audit events that identify caller app, user, tool, conversation, and success/failure without logging message secrets.

Acceptance Criteria

• Tool is disabled unless explicitly allowed by inbound MCP governance.
• Tool cannot target group/public/all-scope contexts.
• Existing conversation writes require ownership or valid access.
• New conversation creation behavior is explicit and tested.
• Content safety and model logging behavior is preserved.
• Rate limits and bounded responses are enforced.
• Tests cover new conversation creation, existing conversation ownership validation, rejection for inaccessible conversations, rate-limit behavior, bounded response behavior, and audit logging redaction.

Notes

Parent: #1013
Depends on the inbound auth/governance foundation and read-only personal tools issues.
Planning doc: docs/explanation/features/MCP_PLUGIN_ROBUSTNESS_PLAN.md
Priority: P1
Size: L