fix(ci): restore macOS test event dispatch

[tamlok]

Failure

The completed macOS run https://github.com/vnotex/vnote/actions/runs/35443251821 failed search, image-host, update-service, bootstrap, and sync-dialog tests.

The process-wide CoreFoundation dispatcher override also changes QThread worker dispatchers. Meanwhile, the offscreen GUI platform still uses a UNIX main dispatcher, so native QtKeychain completions are not drained.

Fix

• Remove the process-wide CoreFoundation override.
• Use a native Qt application on macOS for the eight core/controller tests that perform real keychain operations; retain QCoreApplication on other platforms.
• Run macOS GUI tests with Cocoa, retaining the disposable unlocked test keychain.
• Replace the IO-gate regression's sleep windows and latency thresholds with semaphore-held phases and nonblocking lock probes. Join the worker before asserting, so failures report normally.
• Application behavior and CTest timeouts remain unchanged.

Verification

• Built and ran all 15 affected test executables on Windows: 15/15 passed (116.31 seconds).
• First PR run: all seven original macOS failures passed; Linux and both Windows jobs passed. macOS exposed an unrelated scheduler race in the IO-gate test (238/239 passed): https://github.com/vnotex/vnote/actions/runs/35452733868
• Rebuilt the synchronized IO-gate regression and passed 100 consecutive local runs.
• Final commit 403d0b49: all four CI jobs passed: macOS (tests + signed bundle), Linux, and Windows Qt 5.15 + Qt 6.