Motivation

Two of the numeric checks in validate_buckets_rule raise via assert False in the parse-failure branch. Under python -O asserts get stripped, so a malformed rule like ['tse', 'x', '2', '8'] passes validation silently instead of dying at startup with a useful error.

Repro (extract validate_buckets_rule and call it with the bad rule):

$ python -O repro.py        # unpatched
tse non-float middle: ACCEPTED (no exception)
custom non-numeric:   ACCEPTED (no exception)

Same run against the patched code:

$ python -O repro.py        # patched
tse non-float middle: rejected with ValueError: --generation-tokens-buckets TSE rule parameters must be: [...]
custom non-numeric:   rejected with ValueError: --generation-tokens-buckets custom rule bucket values must be numeric

Modifications

python/sglang/srt/arg_groups/validation_hook.py: replace the two assert False in the parse-failure branches with raise ValueError(...). Messages unchanged. This also matches the rest of the function, which raises ValueError for bad input everywhere else (grpc port, gc_threshold, kv canary, ...).

Verification

Verified locally with the repro above: under python -O both malformed shapes now raise ValueError, and valid rules (tse / default / custom) still pass. No new test file — the diff only changes how the two parse-failure branches report the error.

No GPU needed; this is server-args validation only.

Checklist

• pre-commit ruff gates pass (F401/F821/F823/UP037 + format)
• unit test added and passing (n/a: two-line error-reporting change)
• docs: n/a, error message text unchanged
• benchmarks: n/a, startup validation path only

CI States

Latest PR Test (Base): ❌ Run #40298619201
Latest PR Test (Extra): ❌ Run #40298619121
Latest PR Test (AMD ROCm 10): ❌ Run #40298619180