Add outbound Weixin native voice-message support

[cgf120]

Summary

This PR adds outbound native Weixin voice-message support.

The protocol layer already defines:

• UploadMediaType.VOICE = 4
• MessageItemType.VOICE = 3
• VoiceItem with voice-specific metadata

The README also documents voice_item / SILK-based voice messages.

However, the current outbound implementation only exposes:

• text
• image
• video
• file

Audio files are currently routed through the generic file path instead of being sent as native Weixin voice messages.

What changed

• Added uploadVoiceToWeixin() using UploadMediaType.VOICE
• Added sendVoiceMessageWeixin() to emit MessageItemType.VOICE
• Updated send-media.ts so supported audio files are routed to the voice path instead of always falling back to file attachments
• Preserved file fallback for unsupported audio formats

Notes

• The current implementation routes:
  • .silk → encode_type = 6
  • .mp3 / audio/mpeg → encode_type = 7
  • .ogg / audio/ogg → encode_type = 8
• Unsupported audio formats still fall back to file attachment delivery
• This keeps the existing behavior safe while enabling native voice delivery for supported formats

Why

The protocol and public docs already indicate native voice-message capability, but the outbound implementation was missing the final send/upload path for VOICE.

This PR completes that missing path so outbound behavior matches the documented message schema.

[cgf120]

@pumpkinxing1 HI