fix: subtitle structured output format and remove redundant items field

[guowenfh]

Summary

• Fix --subtitle-format srt not producing actual SRT formatted text in structured output (--json/--yaml)
• Remove redundant items field from subtitle payload — keep only text for AI/human consumers
• Clean up unused normalize_subtitle_items function

Changes

• bili_cli/commands/video.py: Compute formatted text based on subtitle_format and pass to payload
• bili_cli/payloads.py:
  • Add subtitle_formatted_text parameter to normalize_video_command_payload
  • Use formatted text when available, fallback to plain text
  • Remove items field from subtitle payload
  • Remove unused normalize_subtitle_items function
• tests/test_cli.py: Update test to verify timeline format text instead of items

Before

{
  "subtitle": {
    "available": true,
    "format": "srt",
    "text": "plain text...",  // ❌ not SRT format
    "items": [...]            // redundant
  }
}

After

{
  "subtitle": {
    "available": true,
    "format": "srt",
    "text": "1\n00:00:00,121 --> 00:00:05,121\n永不放弃你...\n\n2\n..."  // ✅ actual SRT
  }
}

[guowenfh]

Expected to save about 45% tokens