fix: bound jax and flax metadata scans

[mldangelo-oai]

Summary:\n- stream Flax MessagePack through bounded unpacker limits\n- cap Flax/JAX helper traversal and fail closed on oversize metadata\n- add C171/C173 regressions for oversized/deep fixtures and visible bounded-prefix findings\n\nValidation:\n- Flax + JAX focused suites: 146 passed\n- new C171/C173 regressions: 4 passed\n- ruff check/format, mypy, git diff --check

[mldangelo-oai]

@codex review

[github-actions]

Workflow run and artifacts

Performance Benchmarks

Compared 12 shared benchmarks with a regression threshold of 15%.
Status: 0 regressions, 0 improved, 12 stable, 0 new, 0 missing.
Aggregate shared-benchmark median: 1.250s -> 1.261s (+0.8%).

Workload	Benchmark	Target	Size	Files	Baseline	Current	Change	Status
suspicious-pickle-intake	tests/benchmarks/test_scan_benchmarks.py::test_scan_suspicious_pickle_intake	suspicious-intake	183.8 KiB	4	130.45ms	133.87ms	+2.6%	stable
padded-multi-stream-upload	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_padded_multi_stream_upload	multi_stream_padded	4.1 KiB	1	1.68ms	1.63ms	-2.6%	stable
chunked-upload-stream	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_chunked_upload_stream	chunked_stream	278.2 KiB	1	109.50ms	111.35ms	+1.7%	stable
direct-malicious-upload	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_direct_malicious_upload	malicious_reduce	52 B	1	1.61ms	1.58ms	-1.5%	stable
mixed-model-repository	tests/benchmarks/test_scan_benchmarks.py::test_scan_release_candidate_repository	release-candidate	547.3 KiB	32	401.29ms	405.64ms	+1.1%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_hex]	nested_hex	130 B	1	488.8us	493.9us	+1.1%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_base64]	nested_base64	98 B	1	486.2us	488.3us	+0.4%	stable
single-checkpoint-preflight	tests/benchmarks/test_scan_benchmarks.py::test_scan_single_checkpoint_before_load	single_checkpoint.pkl	183.0 KiB	1	66.26ms	66.55ms	+0.4%	stable
nested-payload-review	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_nested_payload_review[nested_raw]	nested_raw	78 B	1	475.5us	477.5us	+0.4%	stable
warm-cache-rescan	tests/benchmarks/test_scan_benchmarks.py::test_scan_warm_cached_repository_rescan	release-candidate	547.3 KiB	32	71.11ms	71.26ms	+0.2%	stable
clean-training-checkpoint	tests/benchmarks/test_picklescan_benchmarks.py::test_picklescan_clean_training_checkpoint	safe_large	278.2 KiB	1	105.45ms	105.55ms	+0.1%	stable
duplicate-heavy-registry	tests/benchmarks/test_scan_benchmarks.py::test_scan_duplicate_registry_snapshot	registry-snapshot	915.2 KiB	13	361.50ms	361.70ms	+0.1%	stable

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

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Reviewed commit: b6b026a4a8

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Reviewed commit: 25d92a08f2

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