Issues cleanup

[taylorwilsdon]

Had the first quiet day I've had in a long while and ran through some cleanup.

Summary

• Add /health endpoint that pings all connected MCP servers and reports aggregate status (200
  healthy / 503 unhealthy)
• Bypass API key auth for the health endpoint so load balancers and monitoring can reach it
  without credentials
• Support both definitions and $defs in JSON Schema processing for broader MCP tool compatibility
• Gracefully handle unresolved $ref entries by falling back to Any instead of crashing with an
  assertion error

Closes: #293
Closes: #288
Closes: #268
Closes: #225
Closes: #239