Report the codec name, not the decoder name, in probe/1

[rubas-agent]

Before

Exmpeg.probe/1 on main, compared with ffprobe on the same files:

Exmpeg.probe("a.mp3")    -> [{"mp3float", "MP3 (MPEG audio layer 3)"}]
Exmpeg.probe("av1.mkv")  -> [{"libdav1d", "dav1d AV1 decoder by VideoLAN"}]

ffprobe a.mp3            -> mp3,MP3 (MPEG audio layer 3)
ffprobe av1.mkv          -> av1,Alliance for Open Media AV1

Problem

probe/1 reports the name of the first registered decoder, not the codec name. The value changes with the decoders in the FFmpeg build, so a source build and the precompiled NIF can report a different codec for the same file.

After

Exmpeg.probe("a.mp3")    -> [{"mp3", "MP3 (MPEG audio layer 3)"}]
Exmpeg.probe("av1.mkv")  -> [{"av1", "Alliance for Open Media AV1"}]

File	codec before	codec after	ffprobe codec_name
a.mp3	mp3float	mp3	mp3
av1.mkv	libdav1d	av1	av1

h264, aac, vp9, opus, and pcm_s16le stay the same. This is a breaking change for Hex users, and CHANGELOG.md says so under ## Unreleased.

Cause, design, tests

Cause. native/exmpeg_native/src/probe.rs:resolve_codec took name() and long_name() from AVCodec::find_decoder(codec_id).

Design. ffprobe reads both strings from the codec descriptor. rsmpeg has no safe wrapper for avcodec_descriptor_get, so ffi_helpers::codec_descriptor adds one. ffi_helpers is the only module that holds unsafe. The function returns None for an id without a descriptor, and resolve_codec keeps ("unknown", None) for that case, so the contract of Exmpeg.Stream.codec stays. The existing ffi_helpers::codec_name stays as it is, because concat uses it for its error output.

Tests.

• New Rust unit tests in probe.rs: AV_CODEC_ID_MP3 gives ("mp3", Some("MP3 (MPEG audio layer 3)")), AV1 gives av1, H264 gives h264, AAC gives aac, and AV_CODEC_ID_NONE gives ("unknown", None).
• task test:rust: 19 passed.
• task check: exit 0 (fmt, compile, lint, tests, zizmor).
• task test:integration: 104 passed. These tests assert the names that must not change.