fix: ensure appropriate CORS headers for browser based MCP clients

[jappievw]

Description

This PR ensures that the appropriate CORS headers are included both for the .well-known paths as well as the main MCP route. This allows MCP Inspector to connect to an MCP Server exposed by the AI Gateway.

Related Issues/PRs (if applicable)
The CORS headers implemented to support the MCP Inspector were not functioning properly.

For the .well-known/oauth-protected-resource/mcp path CORS headers were returned, see below:

access-control-allow-headers: mcp-protocol-version
access-control-allow-methods: GET
access-control-allow-origin: *

The access-control-allow-headers and access-control-allow-methods are not needed for the protected resources path.

The OPTIONS preflight request for the /mcp route returns an HTTP 401 due to an authorization error, see response headers below:

HTTP/2 401 
content-length: 0
www-authenticate: Bearer error="invalid_token", error_description="The access token is missing or invalid", resource_metadata="https://mcp.host/.well-known/oauth-protected-resource/mcp", scope="scope1 scope2"
access-control-allow-origin: *
access-control-allow-methods: GET
access-control-allow-headers: mcp-protocol-version

There are two problems with this response:

• By design, the CORS preflight calls are unauthenticated. Failing due to auth should not be possible.
• The methods and headers are not sufficient.

Special notes for reviewers (if applicable)
The Envoy SecurityPolicy is now always included to serve the OPTIONS preflight requests and include CORS headers.

This PR has been created with help from Claude (thank you)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.93%. Comparing base (53e58b4) to head (24dfb04).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/mcp_route_security_policy.go	86.20%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2307      +/-   ##
==========================================
+ Coverage   84.88%   84.93%   +0.05%     
==========================================
  Files         144      144              
  Lines       21255    21248       -7     
==========================================
+ Hits        18043    18048       +5     
+ Misses       2132     2126       -6     
+ Partials     1080     1074       -6     

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[nacx]

Thanks! Overall LGTM!

I'm cusious about:

This allows MCP Inspector to connect to an MCP Server exposed by the AI Gateway.

Could you describe what did not work properly? While developing the MCP feature we used the MCP inspector extensively without issues, so I'm wondering what issues you found?

[Hritik003]

Thanks for pointing it out, I remember facing this issue while trying out the PoC with MCP Route Oauth. Had to exclusively add the CORS policy for the well known HTTP Route, for the inspector to able to able to send requests.

LGTM, one minor comment

[Hritik003]

does performance concern here by not setting egv1a1.CORS.maxAge( maxAge tells the browser how long it can cache the preflight response) ?