Improve OAuth diagnostics and error visibility

[technicalpickles]

Description

Improves visibility into OAuth authentication failures by surfacing structured error information through CLI commands and API responses. Users experiencing OAuth issues can now see specific error details (like missing provider parameters) instead of generic connection failures, making it easier to diagnose and report problems.

Key improvements:

• auth status command now displays OAuth-configured servers with clear status indicators (❌ Failed, ✅ Authenticated, ⏳ Pending) and shows specific error messages when authentication fails
• doctor command detects OAuth parameter mismatches and provides actionable resolution guidance with RFC documentation links
• API responses include OAuth configuration details (client_id, scopes, auth_url, token_url) for better debugging
• Structured error parsing extracts parameter names from provider error responses (supports both FastAPI validation errors and RFC 6749 OAuth error format)

User experience example:

Before:

$ mcpproxy auth status
ℹ️  No servers with OAuth configuration found.

After:

$ mcpproxy auth status

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
🔐 OAuth Authentication Status
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Server: slack
  Status: ❌ Authentication Failed
  Auth URL: https://oauth.example.com/authorize
  Token URL: https://oauth.example.com/token
  Error: OAuth provider requires 'resource' parameter

  💡 Suggestion:
     This OAuth provider requires additional parameters that
     MCPProxy doesn't currently support. Support for custom
     OAuth parameters (extra_params) is coming soon.

     For more information:
     - RFC 8707: https://www.rfc-editor.org/rfc/rfc8707.html
     - Track progress: https://github.com/smart-mcp-proxy/mcpproxy-go/issues

Testing

• I have tested these changes locally
• I have added/updated tests that prove my fix is effective or my feature works
• All existing tests pass

Test coverage:

• 7 unit tests for OAuth error parsing covering FastAPI validation errors, RFC 6749 OAuth errors, and edge cases
• Unit test for OAuth config serialization through management service
• Manual testing with auth status and doctor commands confirms diagnostic output

[technicalpickles]

This fixes some parts of what I described in #155 (comment) ... there is a server with oauth but it wasn't showing up.

[Dumbris]

Code Review: Future Enhancement Suggestion

Excellent work on the OAuth diagnostics implementation! The error parsing and structured output are well done.

Future Enhancement

Populate OAuth Metadata URLs from Discovery

Currently auth_url and token_url are hardcoded to empty strings in internal/runtime/runtime.go:1510-1512. The OAuth metadata discovery (via .well-known/oauth-authorization-server) should already have these URLs available. Consider extracting these from the OAuth client's discovered metadata so users can see which endpoints MCPProxy is attempting to authenticate against in the auth status output. This would make debugging OAuth issues even easier by showing the exact authorization and token URLs being used.

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Overall: Great PR, ready to merge! ✅