#12984: fix(signal): fall back to JSON-RPC for health check on signal-cli 0.13+

## Summary Fixes `openclaw doctor` reporting Signal as `failed (unknown) - fetch failed` when using signal-cli 0.13.x, which removed the REST `/api/v1/check` endpoint. Closes #12933 ## Changes - **`client.ts`**: `signalCheck()` now tries REST `/api/v1/check` first, then falls back to a JSON-RPC `version` call at `/api/v1/rpc` if REST fails. Works with both old (<0.13) and new (0.13+) signal-cli versions. ## Testing - Added 5 new tests in `client.test.ts` covering: - REST success (no fallback needed) - REST 404 → JSON-RPC fallback succeeds - REST throws → JSON-RPC fallback succeeds - Both REST and JSON-RPC fail → returns error - Both throw → returns error - All 42 existing signal tests pass 🤖 Generated with Claude (AI-assisted) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR updates the Signal health check to be compatible with signal-cli 0.13+ by attempting the legacy REST endpoint (`/api/v1/check`) first and falling back to a JSON-RPC `version` request against `/api/v1/rpc` when the REST check fails. The change lives in `src/signal/client.ts` (`signalCheck`) and is exercised by a new `src/signal/client.test.ts` suite. `signalCheck` is used by `probeSignal` and the Signal monitor check path, so its success/failure semantics directly affect `openclaw doctor` and runtime health reporting. Main concern: the fallback currently triggers on *any* non-OK REST response, which can mask genuine errors from the REST endpoint on older signal-cli versions (e.g., 401/403/5xx) if JSON-RPC happens to respond. Tests cover the 404/throw scenarios but don’t guard against this behavior change for other statuses. <h3>Confidence Score: 3/5</h3> - This PR is close to safe to merge, but the new fallback logic can change health-check semantics in a way that masks real failures. - The implementation achieves the intended compatibility, but `signalCheck` now treats any non-OK REST response as a reason to fall back and potentially report success if JSON-RPC works, which is a behavioral regression for older signal-cli setups and is not covered by tests. - src/signal/client.ts; src/signal/client.test.ts <!-- greptile_other_comments_section --> <!-- /greptile_comment -->