Harden LegacyMicroVideoProvider with Xiaomi MVIMG ftyp fallback and zero-regex XMP parser

[DichenZhang1]

Summary

Hardens the legacy_micro_video 3P plugin with a zero-regex XMP parser and an ISO BMFF ftyp forward-scan fallback for legacy Xiaomi MVIMG files with inaccurate GCamera:MicroVideoOffset values.

Key Changes

• Zero-Regex XMP Extraction (legacy_micro_video_provider.cc):
  • Replaces std::regex in ExtractInt64Attr with fast absl::string_view parsing supporting both XML attributes (GCamera:MicroVideo="1", Camera:MicroVideo='1') and XML child elements (<Camera:MicroVideo>1</Camera:MicroVideo>).
• Xiaomi MVIMG Offset Drift Fallback (FindMp4StartInTrailer):
  • Scans forward in container.trailer for a valid [u32be box_size]["ftyp"] header when MicroVideoOffset is drifted or preceded by padding after EOI, returning the exact MP4 start offset and length in GetVideoInfo.
  • Excludes trailers carrying 3P proprietary footers (SEFT, jxrs, HiHonor_*, LivePhoto_*, LIVE_*).
• Tests (legacy_micro_video_provider_test.cc):
  • Adds unit and end-to-end extraction tests for XML child-element syntax, Xiaomi MVIMG drifted ftyp offset recovery, and proprietary trailer exclusion.