fix(mcp): only enable /killkillkill under test

[Skn0tt]

/killkillkill was only meant for our graceful-SIGINT test (Windows can't do that via SIGTERM). Anyone who could reach the HTTP port could still trip it with the fixed header.

Gate it on isUnderTest() so production MCP HTTP servers don't expose it.

Fixes #42129

[github-actions]

Test results for "MCP"

1 failed
❌ [firefox] › mcp/annotate.spec.ts:446 › should switch screencast to -s session on show --annotate @mcp-windows-latest-firefox

8049 passed, 1284 skipped

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Merge workflow run.

[github-actions]

Hi, I'm the Playwright bot and I took a first look at the CI failures.

🟢 The one failure is a pre-existing flake — this PR is clear

[firefox] › mcp/annotate.spec.ts:446 › should switch screencast to -s session on show --annotate flakes on its own across the CI history, and this PR only gates the /killkillkill HTTP endpoint behind isUnderTest(), which the annotate/screencast flow never touches.

Details

Overall: one failed test, no real failures caused by this PR.

Pre-existing flake / infra

• [firefox] › mcp/annotate.spec.ts:446 › should switch screencast to -s session on show --annotate (@mcp-windows-latest-firefox) — pre-existing flake, Firefox-heavy. Across the test-results DB this test failed 29 of 637 Firefox runs (~4.5%) and passed the other 608, with 15 more rescued on retry. The failures land on many SHAs this PR can't be responsible for — unrelated PRs (fix(page): collect DOM elements retained by locator hit-target interceptor #41575, feat(devops): add PR CI failure triage bot #41616, chore(protocol): make timeout a protocol-level field #41712, Revert "chore: roll stable test runner to 1.62.0-beta-1784842988000" #42003, test: unflake har-websocket timing assertions #42126, fix(cli): do not lose live sessions to transient pipe connect failures #42128, …) and plenty of push/main commits. On the mcp-windows-latest-firefox bot specifically it's failed 23 of 392 runs. This PR's diff is confined to packages/playwright-core/src/tools/utils/mcp/http.ts (gating /killkillkill on isUnderTest()) and the matching test in tests/mcp/http.spec.ts — nothing in the screencast/annotate path, and the failure reproduces without this change.

_{Triaged by the Playwright bot - agent run}