feat: add serialization for code challenge methods in metadata

[appleboy]

• Add support for serializing supported code challenge methods in AuthServerMetadata

Description

For https://datatracker.ietf.org/doc/html/rfc8414

code_challenge_methods_supported

OPTIONAL. JSON array containing a list of Proof Key for Code Exchange (PKCE) [RFC7636] code challenge methods supported by this authorization server. Code challenge method values are used in the "code_challenge_method" parameter defined in Section 4.3 of [RFC7636]. The valid code challenge method values are those registered in the IANA "PKCE Code Challenge Methods" registry [IANA.OAuth.Parameters]. If omitted, the authorization server does not support PKCE.

	router.GET("/.well-known/oauth-authorization-server", corsMiddleware(), func(c *gin.Context) {
		metadata := transport.AuthServerMetadata{
			Issuer:                            "http://localhost:8080",
			AuthorizationEndpoint:             "https://github.com/login/oauth/authorize",
			TokenEndpoint:                     "https://github.com/login/oauth/access_token",
			RegistrationEndpoint:              "http://localhost:8080/register",
			ScopesSupported:                   []string{"openid", "profile", "email"},
			ResponseTypesSupported:            []string{"code"},
			GrantTypesSupported:               []string{"authorization_code", "client_credentials", "refresh_token"},
			TokenEndpointAuthMethodsSupported: []string{"none"},
			CodeChallengeMethodsSupported:     []string{"S256"},
		}
		c.JSON(http.StatusOK, metadata)
	})

Type of Change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [X] MCP spec compatibility implementation
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update
• [ ] Code refactoring (no functional changes)
• [ ] Performance improvement
• [ ] Tests only (no functional changes)
• [ ] Other (please describe):

Checklist

• [ ] My code follows the code style of this project
• [ ] I have performed a self-review of my own code
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have updated the documentation accordingly

MCP Spec Compliance

• [ ] This PR implements a feature defined in the MCP specification
• [X] Link to relevant spec section: OAuth 2.0 Authorization Server Metadata
• [ ] Implementation follows the specification exactly

Summary by CodeRabbit

• New Features
  • Added support for displaying the code challenge methods supported by the OAuth authorization server.

[coderabbitai[bot]]

Walkthrough

The AuthServerMetadata struct in the OAuth client transport was extended by adding a new optional field CodeChallengeMethodsSupported to represent supported PKCE code challenge methods. No other code or logic changes were made.

Changes

File(s)	Change Summary
client/transport/oauth.go	Added CodeChallengeMethodsSupported []string field to AuthServerMetadata struct with JSON tag code_challenge_methods_supported

[!WARNING] There were issues while running some tools. Please review the errors and either fix the tool's configuration or disable the tool if it's a critical failure.

🔧 golangci-lint (1.64.8)

Error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2 Failed executing command with error: you are using a configuration file for golangci-lint v2 with golangci-lint v1: please use golangci-lint v2

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📥 Commits

Reviewing files that changed from the base of the PR and between 17982da2f257f86ab672df42634b7538df4e1233 and 2d62e3cfb1744cd5d4d4b5912c20058fdd7d2bcf.

📒 Files selected for processing (1)

• client/transport/oauth.go (1 hunks)

✅ Files skipped from review due to trivial changes (1)

• client/transport/oauth.go

✨ Finishing Touches

• [ ] 📝 Generate Docstrings

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[appleboy]

@ezynda3 any feedback?

[appleboy]

Any progress on this?

[appleboy]

@ezynda3 Any feedback for this enhancement?

[appleboy]

Ping @ezynda3