### Motivation
- Harden VP9 RTP payload parsing against malformed/truncated packets to
avoid OOB reads and crashes.
- Fix incorrect assembly of extended `pictureID` and incorrect
consumption of reference indices in flexible mode.
- Add focused unit tests to cover descriptor variants and edge cases so
regressions are caught automatically.
### Description
- Change parser signature to `int parse(const unsigned char *data, int
dataLength)` and replace raw pointer arithmetic with
`readByte`/`skipBytes` helpers that track remaining bytes and enforce
bounds.
- Fix `largePictureID` assembly by concatenating bytes correctly instead
of using `ntohs` arithmetic, and ensure `additionalReferenceIdx` is set
from the parsed byte; also limit flexible-mode reference chain to at
most three references.
- Parse scalability-structure fields (`spatialLayers`, `hasResolution`,
`hasGof`) with bounds checks and correctly read resolution entries and
GOF reference indices; return `-1` on truncation.
- Add an extra safety check in `VP9RtpDecoder::decodeRtp` to verify the
parsed header offset is within payload bounds, and add
`tests/test_vp9_rtp.cpp` with many descriptor/edge-case cases exercising
parsing logic.
### Testing
- Added unit test `tests/test_vp9_rtp.cpp` covering L/F/P combinations,
picture ID variants, reference chains, resolution and GOF parsing, and
truncation cases, and executed it as an automated test.
- The new test binary completed all test cases successfully (all
expected pass/fail outcomes matched).
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[Codex
Task](https://chatgpt.com/codex/cloud/tasks/task_e_6a63e97a4c2c8320aa66d51a746c2e4d)
