build: require PyAV >= 17 and cap requires-python below 3.14

[pengcuo]

Why

Release PRs #265 and #266 fail the unittest job and nothing else. The tree they ship now includes cosmos_framework/data/generator/augmentors/hr_lr_degradation/codec.py, which calls VideoFrame.reformat(threads=...) (PyAV >= 17). This repo lists av with no floor and the lock resolved 16.1.0, so 12 codec tests fail with TypeError: reformat() got an unexpected keyword argument 'threads'. degrade_test.py::test_codec_round_trip_bounds_encoder_and_decoder_threads then leaves its non-daemon thread-count sampler running (the error is raised before stop.set()), pytest never exits after printing its summary at ~36 min, and the job is killed at the 60-minute timeout. Same picture on both days.

What

Mirror the three constraints cosmos-framework-private already has (suggested by @lfengad):

change	private origin
av>=17,<18; python_version < '3.11'	nvidia-cosmos/cosmos-framework-private#94
av>=18,<19; python_version >= '3.11' (what the imaginaire4 images ship)	private e428840
requires-python = ">=3.10,<3.14"	nvidia-cosmos/cosmos-framework-private#92

The cap is not optional: without it uv 0.12.17 cannot re-resolve this lock at all (multi-storage-client 0.44.0 has no aarch64 wheel for the Python 3.14 split), so no dependency bump can land here until it does.

Verification

• uv 0.12.17 (the version setup-uv@v7 installs) reproduces main's uv.lock byte-for-byte on the untouched tree.
• After the change the only (name, version) pairs that move are av 16.1.0 -> 17.1.0 / 18.1.0; every other package is unchanged. The large diff is the dropped 3.14 resolution splits, exactly as in private toml_config: add [job].upload_reproducible_setup, default False for OSS #92.
• uv lock --check passes.

Follow-ups (not in this PR)

• The sampler thread in degrade_test.py should be daemon or wrapped in try/finally so a codec failure can never hang the whole unittest job again; that is an imaginaire4-side change (the file is release-mapped).
• Once this lands, re-run Release 2026-09-20 (i4 4d9b6cfd) #266's GPU Tests (or regenerate it); the failing tests then run on PyAV 18.