fix: add error handling for TikTok parse failures

[Arvuno]

Summary

• Added input validation for TikTok video ID (itemId) to check for empty values and valid numeric format (10-21 digits)
• Added response structure validation to handle unexpected API response types
• Added graceful handling for TikTok API error responses (missing item_info)

Problem

• Invalid TikTok video IDs could cause crashes with unhelpful error messages
• Empty or malformed itemId parameters were not validated before being passed to the crawler
• TikTok API returning success status with missing video data caused unclear failures

Solution

• Added pre-validation of itemId format with regex: 10-21 numeric digits
• Added response type checking (dict expected) with descriptive ValueError
• Added handling for TikTok API returning status_code=0 but missing item_info
• Error messages now clearly indicate whether the issue is:
  • 400: Invalid input format
  • 404: Video not found/unavailable
  • General parse failures

Tests

# Test invalid video ID format
curl "http://localhost:8000/tiktok/web/fetch_one_video?itemId=invalid"
# Expected: 400 with "Invalid video ID format" message

# Test empty video ID
curl "http://localhost:8000/tiktok/web/fetch_one_video?itemId="
# Expected: 400 with "itemId cannot be empty" message

# Test valid video ID (example from docs)
curl "http://localhost:8000/tiktok/web/fetch_one_video?itemId=7339393672959757570"
# Expected: 200 with video data (or 404 if video unavailable)

Risk / Compatibility

• Risk: LOW
• Affects: TikTok web API endpoint only (app API unchanged)
• Backward compatible: Valid requests continue to work as before
• New behavior: Invalid inputs now receive proper error responses instead of generic 400