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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