test: add iterator exhaustiveness tests for FFmpeg components

- Add TestAVCodecIterate_FindsExpectedCodecs verifying 108 codecs:
  - Critical video decoders (h264, hevc, vp9, av1, mpeg2video)
  - Critical audio codecs (aac, mp3, opus, flac, ac3)
  - Subtitle codecs (webvtt)
- Add TestAVMuxerIterate_FindsExpectedFormats verifying 54 muxers:
  - Video containers (mp4, webm, matroska, mov, avi, mpegts, hls)
  - Audio formats and streaming protocols
- Add TestAVDemuxerIterate_FindsExpectedFormats verifying 41 demuxers
- Add TestAVFilterIterate_FindsExpectedFilters verifying 513 filters:
  - Essential video/audio filters and buffer filters
- Add TestAVBSFIterate_FindsBitstreamFilters verifying 37 BSFs
- Add TestAVIOEnumProtocols_FindsExpectedProtocols for I/O protocols

Author: flexiondotorg