fix(client/sse): extract protected resource from eventsource 401

[chrisdickinson]

Previously the SSE connection would always default to the /.well-known/oauth-protected-resource URI, ignoring the resource_metadata portion of the www-authenticate returned in a 401.

Extract the metadata from the initial 401, so RS servers with custom protected resource URIs (as in RFC9728, section 3.1)) continue to work as expected.

Motivation and Context

www.mcp.run hosts multiple MCP server resources. We intend to serve /.well-known/oauth-protected-resource/* for each protected resource, directing connecting clients to the appropriate resource data using www-authenticate. We do this so that we can return the resource attribute matching the server resource accessed. This works as expected with the HTTP streaming transport, but fails with the current SSE server.

The SSE server currently does not intercept the www-authenticate header to extract the resource_metadata URI. As a result, it requests the top-level protected resource document
whose resource attribute does not match the resource being accessed by the client. This causes the client to fail.

This change aligns the SSE www-authenticate behavior with the HTTP streaming transport.

How Has This Been Tested?

I've tested these changes against our development server.

Breaking Changes

This should be largely non-breaking -- users that provide their own fetch implementation will continue to see it called, but they may see additional headers (via this._commonHeaders().)

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

[ihrpr]

Thank you!