fix(oauth): rfc8414 should judement the response_types

[jokemanfire]

response_types_supported should be judement while try to do 'Authorization Code Flow'

Motivation and Context

according to rfc8414

How Has This Been Tested?

No

Breaking Changes

auth server should be more standardized.

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

[jokemanfire]

response_types_supported
REQUIRED. JSON array containing a list of the OAuth 2.0
"response_type" values that this authorization server supports.
The array values used are the same as those used with the
"response_types" parameter defined by "OAuth 2.0 Dynamic Client
Registration Protocol" [RFC7591].

[jokemanfire]

Should the option here be removed for compatibility with rfc8414?

[Copilot]

Pull Request Overview

This PR implements RFC 8414 compliance by adding validation for the response_types_supported field in OAuth 2.0 authorization server metadata to ensure servers support the "code" response type used in Authorization Code Flow.

• Adds response_types_supported field to AuthorizationMetadata struct
• Implements validation in both registration and authorization flows to check for "code" support
• Updates example server to include the required field and simplifies metadata creation using Default

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
crates/rmcp/src/transport/auth.rs	Adds response_types_supported field and validation logic for RFC 8414 compliance
examples/servers/src/complex_auth_sse.rs	Updates example server to include response_types_supported field

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[jokemanfire]

@alexhancock@4t145 pls check it .

[jokemanfire]

@alexhancock If possible, please merge this first. This is also explicitly mentioned in the specification.
https://modelcontextprotocol.io/specification/2025-11-25/basic/authorization