Guard test backend ready channel against double close

[jhrozek]

Summary

TestCreateMCPClient_ContinuousListeningGatedOnSink shares one listChangedTestBackend across two t.Parallel() subtests, each of which connects its own MCP client. The backend's OnRegisterSession hook did an unguarded close(b.ready) — but that hook fires once per registered session, not once per backend. When both clients registered, the channel was closed twice and the test binary died with panic: close of closed channel.

Because the panic aborts the whole binary, it surfaced as a failure of whatever unrelated test happened to be mid-run — TestInitAndQueryCapabilities_FatalErrors, TestParsingMiddlewareWithRealMCPClients, a pkg/ignore test — which made it look like several different flakes rather than one bug.

The fix guards the close with a sync.Once, so the helper is correct no matter how many sessions register. The alternative (give each subtest its own backend) fixes this call site but leaves the helper fragile for the next test that shares one.

Closes #6362

Type of change

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

Test plan

• Unit tests pass

Reproduced first, then verified the fix. The panic needs varied GOMAXPROCS; it does not fire at default parallelism.

Before (fires within ~3 iterations):

go test -race -count=40 -cpu=1,2,8 ./pkg/vmcp/session/internal/backend/
...
panic: close of closed channel
	pkg/vmcp/session/internal/backend/mcp_session_test.go:236
	mcpcompat/server.(*Hooks).registerSession
	mcpcompat/server.(*MCPServer).registerAndSync
	go-sdk/mcp.(*ServerSession).initialized

After: 4 × -count=40 -cpu=1,2,8 (480 runs) all green. A separate 720-run check during investigation was also clean.

Plain -count=60 at default -cpu stayed green both before and after, which is why this only showed up on CI runners.

Does this introduce a user-facing change?

No — test-only change.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.04%. Comparing base (7a0c0a5) to head (a132ede).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6363      +/-   ##
==========================================
+ Coverage   72.97%   73.04%   +0.06%     
==========================================
  Files         742      742              
  Lines       78398    78491      +93     
==========================================
+ Hits        57208    57330     +122     
+ Misses      17201    17153      -48     
- Partials     3989     4008      +19     

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