v0.28.2 — layout-agnostic proxy-proof health exemption

Patch on 0.28.1.

test_health_and_options_exempt asserted health at {prefix}/health, which is where the Python worker mounts it. Rust mounts health at the root even when RPC is under a prefix, so the assertion failed there.

It now probes both candidates and requires only that health answers somewhere without a proof. Asserting the absence of 401 across both would fail on Python for the opposite reason — a port that authenticates before routing returns 401 for any path it does not serve.

Same class as 0.28.1: the shared conformance suite cannot assume one mount layout. Verified green (22/22) against the Python, Go and Rust runners.