Fix default Dock icon dynamic rendering

[zzj3720]

Summary

• Fixes Default Dock icon does not follow macOS 26 Dark or Tinted appearance #37 by clearing the runtime NSApp.applicationIconImage override when the default app icon is selected, letting macOS render the bundled Icon Composer icon dynamically for Dark/Tinted Dock styles.
• Keeps custom icon behavior unchanged: alternate icons still set the bundle icon and runtime Dock image.
• Adds unit coverage for default/custom icon application and a debug E2E report path for Dock-icon launch checks.

Validation

• zsh DevKit/Scripts/swift_format_lint.sh --strict
• Fresh minimachine (macOS 26.2) build via macos/DevKit/skill/setup_bridge_on_machine.sh --branch codex/issue37-dock-icon --repo-dir ~/openbridge
• Fresh minimachine Dock smoke: switched Appearance → Icon & widget style to Dark and Tinted, relaunched OpenBridge with default icon + Dock icon enabled, and confirmed the debug E2E report wrote appIcon=default and runtimeIconOverrideCleared=true.
• Attempted targeted xcodebuild test -only-testing:OpenBridgeUnitTests/AppIconTests on the minimachine; app/tests built, but the run was blocked by the machine's com.apple.testmanagerd.control helper not running (Failed to establish communication with the test runner).