fix(rtc): use the right plane layout for I420A, I422 and NV12 frames

[ANDROIDHASSAN]

Two helpers in video_frame.py compute the layout of a packed frame, and three formats were off:

• _get_plane_length sized the chroma of I420A and NV12 as chroma_width * chroma_width instead of chroma_width * chroma_height. _from_owned_info copies that many bytes out of FFI memory for every frame a VideoStream delivers in that format, so a landscape frame is copied past the end of its buffer and a portrait one comes back short. For 640x480: NV12 512000 bytes instead of 460800, I420A 819200 instead of 768000.
• _get_plane_infos started the I422 V plane at u.data_ptr + u.size + u.size, one U plane too late, which is the end of the buffer. get_plane(2) reads from there, and _proto_info sends that pointer to the FFI on VideoSource.capture_frame.

The fix is the three formulas; I420, I444 and I010 already matched.

Tests in tests/rtc/test_video_frame.py:

• test_planes_are_packed_and_match_the_frame_length: for every planar format and 640x480, 480x640 and 5x3, the plane sizes match the format, the planes follow each other with no gap, and _get_plane_length equals their total. Fails on main for I420A, I422 and NV12 at every size.
• test_frame_from_the_ffi_copies_exactly_the_frame[I420A|NV12]: _from_owned_info on a 640x480 buffer copies exactly the frame (the buffer is allocated larger, so main over-reads safely and the test just fails).
• test_get_plane_returns_each_i422_plane: get_plane(0..2) returns the Y, U and V bytes of an I422 frame. Fails on main.

Ran tests/rtc against the published livekit wheel with video_frame.py (and main's audio_mixer.py) swapped in: the new tests are the only difference between main and this branch (test_agc/test_apm/test_resampler fail identically on both here, from missing local fixtures). ruff format --check, ruff check (0.15.4) and mypy on the module pass.

Scope: left alone, each separate from these formulas: VideoFrame.__init__ does not check len(data) against _get_plane_length, so a short buffer can still be over-read by get_plane/capture_frame; _from_owned_info assumes one packed buffer rather than reading info.components; get_plane accepts a negative index.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.