discovery(frontend): useProbeBackedHealth JSDoc claims null-on-error but implementation returns buildUnavailable() #10119

Found during #9866 (U7 #9926)

autobot-frontend/src/composables/useProbeBackedHealth.ts:76 — the JSDoc says the factory returns null on fetch error ("null-on-fetch-error"), and the signature is () => Promise<R | null>. But the implementation's catch block returns buildUnavailable('Service unavailable') and never actually returns null.

Impact

Consumers coded against the documented null branch (e.g. if (res === null)) will never hit it — the error path silently yields an 'unavailable' health object instead. Stale docstring vs real behavior.

Needed

• Either update the JSDoc + return type to reflect that it always resolves to R (never null), or restore the null-on-error path if that was the intended contract.

Refs

• autobot-frontend/src/composables/useProbeBackedHealth.ts:76
• Documented in docs/developer/FRONTEND_COMPOSABLES.md (docs(frontend): document useProbeBackedHealth composable #9866)