fix(runtime): classify expired probe budget as timeout

[liugddx]

Summary

• classify an already-exhausted shared OpenCode fallback budget as timeout
• avoid starting a provider request after the strict deadline has elapsed
• remove the test assumption that at least one probe must start before a shared deadline expires

Why

If the process is descheduled for the full 40ms test budget before the first fallback iteration, no probe starts and lastFailure remains empty. The old fallback then reported provider_unavailable, even though the only known fact is that the shared deadline expired. This caused a CI flake and exposed an incorrect production edge classification.

Validation

• npm --workspace @maka/core run build
• npm --workspace @maka/runtime run build
• target deadline test repeated 30 times
• node --test packages/runtime/dist/__tests__/opencode-free-anonymous.test.js (7 pass)
• Biome and git diff --check

[liugddx]

@Astro-Han

[Astro-Han]

Thanks — this is a clean, minimal reclassification. We traced the reachability: on the dominant path (all candidates fail, last one aborted) the old code already returned timeout, so the real change is the unreachable-in-production index-0 budget-expiry corner, and every reachable outcome is field-for-field identical to the old behavior. Consumers (connections:test status patch, runtime-host summary, redaction of 'Fetch timeout') all map consistently; no retry/metric consumers exist. 7/7 + 10/10 + 19/19 green.

Two optional notes:

• The deleted assert.ok(requestedModels.length > 0) (opencode-free-anonymous.test.ts:244,254-255,273) was the only assertion that could distinguish 'probes were actually issued' from 'no probe was ever issued'. Under the new semantics the test would pass even if the loop never fired a single request. Keeping it (it still passes on a normal machine) or an equivalent counting assertion would keep the deadline test load-bearing.
• The lastFailure ?? timeout fallback at test-connection.ts:171 is dead code (the candidate list is statically non-empty and index-0 expiry is pre-returned) — harmless, could be simplified.

Merging.