fix(server): validate sampling params and rebuild timeout at the API layer

[Artemowka22]

Summary

1. Invalid sampling silently reaches the engine. resolve_sampling forwards arbitrary client floats: temperature=-3, NaN, inf, top_p=0, top_p=7, top_k=0 all become a SamplingParams and die (or misbehave) deep in the sampler instead of failing the request. Fix: after checkpoint defaults are filled, validate temperature (finite, ≥ 0), top_p ((0, 1]), top_k (−1 or ≥ 1) and raise ValueError in the same style as the existing max_tokens check — every adapter already maps that to a clean HTTP 400.

2. CacheRebuildRequest.timeout is unbounded. A negative/NaN/huge timeout reaches asyncio.wait_for on the rebuild future; inf parks the handler forever. Fix: pydantic bound 0 < timeout ≤ 3600.

The audit that surfaced these (agisota/freetoken-mlx, docs/AUDIT.md, defects 3–4) also reported a TOCTOU race on concurrent /v1/cache/rebuild; current upstream has already restructured that path (single synchronous section from the pre-flight checks to maintenance_state = "rebuilding"), so only the timeout bound is ported.

Test plan

• parametrized invalid/boundary sampling matrix on /v1/chat/completions in tests/server/test_openai_api.py — 400 with the offending parameter named; boundary values temperature=0, top_p=1, top_k=-1, top_k=1 accepted
• rebuild timeout bounds in tests/server/test_rebuild_maintenance.py
• full tests/server/ suite passes