fix: make shutdown tests reliable on windows-latest

[triuzzi]

Problem

tests/shutdown.test.ts flakes on windows-latest, for two distinct reasons.

1. The stdin-EOF budget is too tight for Windows teardown. On a recent main run:

✖ exits within budget on stdin EOF after Chrome launch
  AssertionError: stdin-EOF shutdown took 6451ms (budget 3000ms)
ERROR: The process with PID 5496 (child process of PID 5024) could not be terminated.

The graceful path tears down the Chrome process tree via taskkill /T, far slower than POSIX teardown (~6.5s observed vs <500ms) and noisy with benign "could not be terminated" retries, so the flat SHUTDOWN_BUDGET_MS = 3000 is exceeded. --test-concurrency=1 (#2205) reduced overlap but doesn't change this budget.

2. The signal tests don't exercise graceful shutdown on Windows. child.kill('SIGTERM'|'SIGINT'|'SIGHUP') maps to TerminateProcess on Windows (no POSIX signals), so the server's handlers never run — those three tests hard-kill instantly, assert nothing about graceful shutdown, and leak the Chrome process tree.

Fix

• stdin-EOF budget → 8000ms on Windows (3000ms elsewhere). 8000ms is above the real ~6.5s teardown but below the production force-exit backstop in chrome-devtools-mcp-main.ts (an unref'd setTimeout(process.exit(0), 10000)), so a genuine hang rescued by that backstop (exiting ~10s) is still caught.
• Skip the three signal tests on Windows — child.kill(signal) can't trigger the graceful handlers there. stdin-EOF remains the cross-platform graceful-shutdown coverage.
• Reuse the shared IS_WINDOWS from src/daemon/utils.ts.

Linux/macOS behavior is unchanged.

Related: #2137

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