Deflake real-backend forwarding tests

[JAORMX]

Summary

Why: TestForwarding_*_RealBackend (pkg/vmcp/server/forwarding_realbackend_integration_test.go) intermittently time out on CI waiting for a forwarded, server-initiated notification (#5962). waitNotification used a hardcoded time.After deadline (5s originally, 15s after #5941) inside the per-test context — a two-deadline design where a timer shorter than the context flaked under the full-suite parallel -race load (the async backend → vMCP → downstream relay can take several seconds), while a longer timer would have masked a genuine hang. The 15s bump proved insufficient (TestForwarding_Logging_RealBackend timed out at 15.09s).

What:

• waitNotification now selects on the caller's ctx.Done() instead of its own timer (and reports ctx.Err() on timeout) — a single source of truth for the deadline.
• Every real-backend forwarding test shares one generous const forwardingRealBackendTimeout = 60 * time.Second, replacing the per-test 20s/30s literals. 60s is well under the 10m go test global timeout, so a real hang still fails fast with a clear error rather than blocking the whole suite.
• notifCh is left buffered at 8; each test emits and reads a single notification, so drop-on-full is not a realistic flake source.

Closes #5962.

Type of change

• Bug fix

Test plan

• go test -race -count=5 -run TestForwarding_ ./pkg/vmcp/server/ — passes 5×.
• task lint 0 issues; task build succeeds.

Does this introduce a user-facing change?

No. Test-only change.

Special notes for reviewers

• Reviewed for concurrency/test-adequacy: waitNotification's t.Fatalf runs on the test goroutine (called directly, not in a spawned goroutine), and each defer cancel() only fires on function return, so tying to the caller ctx is sound.
• Minor, deliberate: the two negative-path tests (withHandlers=false) that assert require.NotErrorIs(err, context.DeadlineExceeded) now use 60s instead of 20s. They fail near-instantly in practice, so the widened window is a negligible loss of timing sensitivity, kept for the single-const simplicity.
• Follow-up to Deflake vMCP forwarding notification wait #5941, which raised only the Progress variant's deadline to 15s.

Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.84%. Comparing base (97a9dfa) to head (c51fbd5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5963      +/-   ##
==========================================
- Coverage   71.84%   71.84%   -0.01%     
==========================================
  Files         708      708              
  Lines       72811    72811              
==========================================
- Hits        52311    52310       -1     
- Misses      16765    16767       +2     
+ Partials     3735     3734       -1     

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