feat: bound accessibility snapshot requests to avoid indefinite hangs

[mykola-mokhnach]

Summary

Fixes #1210: an app that stops answering accessibility requests could
previously block the whole WDA server forever, since XCTest offers no
bounded timeout for the underlying accessibility snapshot request that
backs most element/attribute lookups (including .identifier, active-app
detection, etc.).

• Adds a new accessibilityDeadline setting/capability (NSTimeInterval,
  disabled by default). When set to a value > 0, a swizzle on
  -[XCAXClient_iOS requestSnapshotForElement:attributes:parameters:error:]
  first confirms the target app's main run loop is genuinely responsive
  (via the private notifyWhenEventLoopIsIdleForApplication: API, wrapped
  in FBXCAXClientProxy) before letting the real snapshot request proceed.
  If the app doesn't confirm responsiveness within the deadline, the
  request is aborted with a clear "illegal/unresponsive application state"
  error instead of risking an indefinite hang.
• XCTest internally retries a failed snapshot request several times before
  giving up. To avoid multiplying the configured deadline by the retry
  count, a short-lived per-pid cache remembers "this app was just confirmed
  unresponsive," so those retries fail fast instead of each re-running the
  full wait.
• handleActiveAppInfo: (GET /wda/activeAppInfo) is made resilient to
  .identifier returning nil (which can legitimately happen once the new
  deadline aborts the underlying fetch) - it now reports "unknown" for
  the app name instead of crashing.

Test plan

• New integration test FBConfigurationTests.testAccessibilityDeadlineAbortsSnapshotRequestForDeadlockedApp
  freezes the IntegrationApp fixture's main thread (Deadlock app button,
  reworked to sleep instead of self-deadlock so it doesn't trip the OS
  watchdog) and asserts that a snapshot request against it aborts within
  bounds instead of hanging. Skipped on CI (CI env var) since it
  deliberately freezes the app for ~20s.
• Verified manually end-to-end against a temporary freeze harness: with
  accessibilityDeadline set, a frozen app now fails gracefully in a few
  seconds; with it unset (default), original unbounded behavior is
  preserved unchanged.
• WebDriverAgentLib and the new test target build and pass locally.

[Dan-Maor]

I will review it and run some tests tomorrow, very hectic day today.