Thread VirtualMCPServer name into Cedar authz middleware

[jhrozek]

Summary

• Cedar authorization policies reference the actual VirtualMCPServer name as the resource entity (e.g. MCP::"my-vmcp"). The factory hard-coded the string "vmcp" when calling authz.CreateMiddlewareFromConfig, so resource entities never matched and Cedar's default-deny fired on every request — 403 for all principals regardless of policy.
• Fix by adding a serverName parameter to NewIncomingAuthMiddleware and newCedarAuthzMiddleware, passing vmcpCfg.Name at the serve.go call site.
• Add an empty-string guard in newCedarAuthzMiddleware so a misconfigured (empty) server name fails loudly at startup rather than silently mismatching all resource-scoped policies.

Fixes #5428

Type of change

• Bug fix (non-breaking change which fixes an issue)

Test plan

• Added regression test TestNewIncomingAuthMiddleware_ServerNameThreaded that compiles a Cedar permit policy scoped to resource in MCP::"my-production-vmcp", calls NewIncomingAuthMiddleware with serverName = "my-production-vmcp", and asserts the request is permitted (200).
• Negative sub-test with serverName = "wrong-server" asserts 403 — verifies that the server name is actually threaded through to Cedar evaluation, not hardcoded.
• task test passes on the affected package.

Does this introduce a user-facing change?

Yes: VirtualMCPServer Cedar authorization now correctly evaluates resource-scoped policies using the server's actual name. Previously all Cedar-gated requests returned 403 when policies used resource in MCP::"<name>" scoping.

Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.85%. Comparing base (a785995) to head (a57f278).

Files with missing lines	Patch %	Lines
pkg/vmcp/auth/factory/incoming.go	60.00%	1 Missing and 1 partial ⚠️
pkg/vmcp/cli/serve.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5448      +/-   ##
==========================================
- Coverage   68.85%   68.85%   -0.01%     
==========================================
  Files         634      634              
  Lines       64437    64439       +2     
==========================================
- Hits        44371    44369       -2     
- Misses      16789    16792       +3     
- Partials     3277     3278       +1     

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