Don't trap on an unknown AVSpeechSynthesisVoiceGender

[ronjb]

The crash

getVoices reads voice.gender.stringValue for every installed voice. AVSpeechSynthesisVoiceGender is an Objective-C enum, so AVSpeechSynthesisVoice.speechVoices() can hand back a voice whose gender is a case the app was not compiled against. The switch in stringValue covers only .male, .female and .unspecified and has no @unknown default, so Swift traps in _diagnoseUnexpectedEnumCaseValue and the whole app goes down.

Here is a production crash from an iPad Air 2 on iOS 15.8.8, reporting rawValue: 3:

EXC_BREAKPOINT: Fatal error: unexpected enum case 'AVSpeechSynthesisVoiceGender(rawValue: 3)'

  at SwiftFlutterTtsPlugin.getVoices (SwiftFlutterTtsPlugin.swift:346)
        voiceDict["gender"] = voice.gender.stringValue
  at AVSpeechSynthesisVoiceGender.stringValue.getter (SwiftFlutterTtsPlugin.swift:502)
      case .unspecified:
  at _diagnoseUnexpectedEnumCaseValue<T>
  at _assertionFailure

Two things make this worse than a normal bug:

• It is not recoverable from Dart. A Swift fatalError is a process trap, not a PlatformException, so wrapping flutterTts.getVoices in try/catch does nothing.
• It is not transient. The offending voice stays installed, so every fetch of the voice list kills the app for that user.

The fix

Add @unknown default returning "unspecified", which is already the documented fallback on the Dart side. The macOS plugin carries an identical extension and gets the same change.

Verified with swiftc -typecheck -warnings-as-errors against the iOS SDK: the enum is non-frozen, so the new case compiles clean with no "will never be executed" warning.