test(e2e): fix no-op security injection test via frames[] traversal

[ochafik]

Summary

The cross-app message injection test at security.spec.ts:313 has been a no-op since it was added in 2411c71. This PR makes it actually exercise the event.source check in PostMessageTransport.

The bug

The old test tried to reach the inner app iframe via:

outerIframe.contentDocument?.querySelector('iframe')

But host runs on :8080 and the sandbox iframe on :8081, so contentDocument is null cross-origin. The ?. short-circuited to undefined, the early return at :341 fired, and postMessage never executed. The test passed vacuously.

Ironically, the test ~60 lines above it (added in the same commit) explicitly asserts contentDocument === null:
https://github.com/modelcontextprotocol/ext-apps/blob/2411c71a/tests/e2e/security.spec.ts#L247

Why frames[] works

Per HTML spec, window.frames[] is one of the few properties that remains accessible across origins — it's the mechanism by which cross-origin windows can postMessage each other at all. This is also the actual attack path from the threat model comment at :309: a malicious sibling app reaches window.parent.parent.frames[i].frames[0].postMessage(...). The message does get delivered. PostMessageTransport's event.source !== window.parent check (message-transport.ts:77) is the only defense, and this test now actually verifies it.

Sentinel against recurrence

page.evaluate() now returns "POSTED" / "UNREACHABLE" and we assert on it. If frame traversal ever breaks again (hierarchy change, etc.), the test fails at the sentinel instead of silently passing.

Other changes

• Actually assert rejectionLogs.length > 0 (was captured but never checked — see old :359 comment "ideally we see rejection")
• expect.poll() instead of waitForTimeout(500)
• Reuse existing captureConsoleLogs helper
• Drive-by: fixed misleading comment at :293 — it claimed "Outer frame should NOT have allow-same-origin" but the assertion checks allow-top-navigation. The outer frame does have allow-same-origin (implementation.ts:167); isolation comes from the separate port.

Verification

npx tsc --noEmit clean. Not run locally — Playwright can't run headless outside docker on my machine. CI green (19/19 checks).

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commit: 74c4784