GHSA-48jh-3gj7-fg8v

### Summary

The fix for `GHSA-rwxx-mrjm-wc2m` ("ReDoS via `structured_outputs.regex` compiled without timeout") wrapped the regex compile in the **xgrammar** and **outlines** backends with `compile_regex_with_timeout` (and, for outlines, `validate_regex_is_buildable`). The **lm-format-enforcer** backend was left unguarded: it compiles the attacker-supplied regex with no timeout and no buildability check. A single request with a catastrophic regex hangs the structured-output compile step and stalls the engine worker (denial of service).

### Affected code (HEAD d6d39c1)

`vllm/v1/structured_output/backend_lm_format_enforcer.py`: - line 110: `character_level_parser = lmformatenforcer.RegexParser(grammar_spec)` — builds an `interegular` FSM from the attacker regex synchronously, **no timeout**. - line 155: `validate_structured_output_request_lm_format_enforcer` returns immediately on `if so_params.regex:` — **no validation**.

Sibling backends that WERE patched by GHSA-rwxx: - `backend_xgrammar.py:92` → `compile_regex_with_timeout(...)`. - `backend_outlines.py:65` → `compile_regex_with_timeout(...)` (plus `validate_regex_is_buildable`).

lm-format-enforcer uses the same `interegular` DFA-construction primitive the advisory cites for the outlines backend.

### Reproduction (runtime-verified against the sink)

The sink `lmformatenforcer.RegexParser(<regex>)` was exercised directly (this is exactly what the backend calls):

``` baseline '[0-9]{3}' -> 0.0002 s attacker '(a{1,300}){300}' -> DID NOT COMPLETE in 20 s (one core pegged at 100% in interegular FSM construction) ```

End-to-end: start `vllm serve <model> --structured-outputs-config '{"backend":"lm-format-enforcer"}'`, then `POST /v1/completions` with `{"structured_outputs":{"regex":"(a{1,300}){300}"}, ...}`. The request never returns; because grammar compile runs in the engine's structured-output path, concurrent requests stall = worker-level DoS. The identical request against the outlines backend is bounded by `compile_regex_with_timeout` and returns a clean error.

### Impact

Unauthenticated denial of service (vLLM ships with no authentication by default). One request pegs a CPU core and blocks the structured-output engine path.

**Reachability precondition:** the operator must have selected `backend=lm-format-enforcer` via `--structured-outputs-config` (the default is `auto` → xgrammar). This is the same opt-in tier as the outlines backend that GHSA-rwxx already covered.

### Suggested remediation

Route the lm-format-enforcer regex compile (`backend_lm_format_enforcer.py:110`) through the same `compile_regex_with_timeout` guard already applied to the xgrammar and outlines backends, and reject un-buildable / oversized patterns in `validate_structured_output_request_lm_format_enforcer`.