Follow HTTP redirects in transparent proxy

[gkatz2]

Summary

• MCP clients cannot handle HTTP redirect responses (they expect JSON-RPC), so when a remote server redirects, the client silently fails. The transparent proxy now follows redirects in its forward method before returning the response to the client.
• Adds an isRedirectStatus helper that handles 301, 302, 307, and 308. The HTTP method and body are preserved across redirects via req.Clone(). A maxRedirects limit of 10 prevents infinite loops.
• Logs at WARN level on each redirect to nudge operators toward fixing the server URL.

Fixes #4453

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

Tested with a local HTTP server returning 308 redirects to a running MCP server (design-system-mcp). Verified:

• initialize call succeeds through the redirect
• Proxy logs show the WARN message with from/to URLs and redirect count

Does this introduce a user-facing change?

Remote MCP servers behind HTTP redirects now work. Previously, the proxy passed redirect responses to the client, causing silent failures. No configuration needed — redirect following is automatic.

Special notes for reviewers

• The redirect logic lives in forward rather than RoundTrip so that session tracking, 401 handling, and DELETE cleanup in RoundTrip only see the final response after all redirects are resolved.
• 303 (See Other) is excluded from isRedirectStatus because it explicitly requires changing the method to GET, which would break JSON-RPC.

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.61%. Comparing base (11cd266) to head (0eed864).

Files with missing lines	Patch %	Lines
...g/transport/proxy/transparent/transparent_proxy.go	78.94%	5 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4454      +/-   ##
==========================================
+ Coverage   69.57%   69.61%   +0.04%     
==========================================
  Files         489      489              
  Lines       50193    50228      +35     
==========================================
+ Hits        34920    34965      +45     
+ Misses      12590    12575      -15     
- Partials     2683     2688       +5     

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