feat(live): add support for voice activity start and end events

[wuliang229]

Summary

Adds support for visualizing Voice Activity Start and Voice Activity End events in ADK Web's chat event list.

In bidirectional live sessions (e.g. Gemini Live), the server sends voiceActivity events (ACTIVITY_START and ACTIVITY_END) with optional audio offsets indicating when speech begins and ends in the audio stream. Previously, these events had no UI chips and rendered as empty node rows in the chat timeline.

Changes

• Core Types (src/app/core/models/types.ts):
  • Added VoiceActivity interface with properties voiceActivityType and audioOffset.
  • Added voiceActivity?: VoiceActivity to LlmResponse (inherited by Event).
• Event Content Component (src/app/components/event-content/event-content.component.html):
  • Added chips for Voice Activity Start (icon mic) and Voice Activity End (icon mic_off).
  • Added concise tooltip content (e.g. Started at 1.5s / Ended at 3.2s, or Started / Ended).
• Chat Component (src/app/components/chat/chat.component.ts):
  • Ensured isEmptyMetadataEvent does not drop events carrying voiceActivity.
• Tests:
  • Added unit test in src/app/components/chat/chat.component.spec.ts for isEmptyMetadataEvent.
  • Created src/app/components/event-content/event-content.component.spec.ts covering Start, End, and offset fallback rendering.