feat: add privacy-safe MCP tool telemetry

[leocuisine]

Summary

• Emit one structured diagnostic event for each top-level remote MCP tool call.
• Record the tool name, upstream REST status or statuses, bounded error class, and latency.
• Exclude tool arguments, prompts, query text, response bodies, IP addresses, and session identifiers.
• Document the short-lived, diagnostic-only Workers Logs boundary.

Verification

• npm run check in worker/ — passed (typecheck and 7 tests)
• npx --no-install prettier --check . — passed
• git diff --check — passed

Deployment note

The telemetry will become active after this PR is merged and the MCP Worker is deployed.