Consolidate the two "Select Microphone" commands into one

[Copilot]

The command palette exposed two commands that both pick a microphone and persist to the same AgentsVoiceStorageKeys.MicrophoneDevice storage key: Chat: Dictate: Select Microphone and Voice: Select Microphone. The Voice Mode variant was fully redundant — the dictation command is already the one wired into both the dictation and Voice Mode mic-button context menus.

Description

• Removed agentsVoice.selectMicrophone (Voice: Select Microphone) and its now-unused IQuickInputService import in agentsVoice.contribution.ts.
• Generalized the surviving workbench.action.chat.selectSpeechToTextMicrophone:

  • Retitled "Dictate: Select Microphone" → "Select Microphone" (still under the Chat category → Chat: Select Microphone).

  • Broadened the precondition so it stays discoverable when either feature is on:

    precondition: ContextKeyExpr.or(
        ChatSpeechToTextConfigured,
        ContextKeyExpr.equals('config.agents.voice.enabled', true)
    ),

The kept command uses the canonical buildMicrophoneOptions normalization (including the "System default" entry that clears the stored device), so no behavior is lost for Voice-Mode-only users.

• Fixes Do we need 2 commands to set the microphone? #328790