Add support for pre-configured OAuth client credentials

[jancurn]

Summary

This PR adds support for servers that don't support dynamic OAuth client registration by allowing users to provide pre-configured client credentials (client ID and optional client secret) during authentication.

Key Changes

• CLI Enhancement: Added --client-id and --client-secret options to the mcpc login command to accept pre-configured OAuth credentials
• OAuth Flow Updates: Modified performOAuthFlow() to accept optional client credentials and skip dynamic client registration when credentials are provided
• Keychain Management: When client credentials are provided, the flow now stores them in the keychain instead of deleting existing client info
• Provider Configuration: Updated OAuthProvider to:
  • Accept clientCredentials in OAuthProviderOptions
  • Use client_secret_post authentication method when a client secret is provided (confidential client mode)
  • Skip dynamic registration and use pre-configured credentials directly
• Validation: Added validation to ensure --client-secret is only used with --client-id

Implementation Details

• When client credentials are provided, they bypass the dynamic client registration flow entirely
• The authentication method automatically switches from none (public client) to client_secret_post (confidential client) when a client secret is present
• Pre-configured credentials are stored in the keychain for future use, maintaining consistency with the existing token management system
• The change is backward compatible - existing flows without credentials continue to work as before

https://claude.ai/code/session_01SQSYrXbVmLATkqq59zkSY8