fix: quick oauth flow could not transition to token_request due to missing server metadata

[kamal]

Motivation and Context

I was testing the new auth debugger and noticed the Quick OAuth Flow would fail to transition due to it missing the server metadata.

Fixed by:

1. Adding explicit metadata discovery before the auth call
2. Saving the metadata to the provider
3. Adding tests for the quick OAuth flow to prevent future regressions

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[kamal]

@pcarleton great work on the Auth Debugger. I was taking it for a spin today when I noticed this issue. Once this is fixed, I noticed that the quick flow still waits for me to press continue on the "Token Request" step (now it passes - previously it would error with unable to transition). Is this a deliberate choice or should it complete all steps to completion?

[pcarleton]

Thanks for this @kamal

re: quick flow stopping before token fetching, it's known but not necessarily intentional. It's a side effect of the callback handling being shared between the two flows.

My ideal is that the quick flow goes through the same state machine as the step-by-step flow, but doesn't ask confirmation between steps, but I haven't found time to get that working.

Your change adds more special handling for the quick flow which we'd delete in that case, but that seems fine to add until we get it on the state machine.