restic: tighten RunCheck lock sniff + RunStats zero-snapshot test

Narrow the LockPresent predicate from bare "locked" (too broad) to
"stale lock" and "already locked" — the two phrases restic actually
emits. Replace TestRunCheckParsesLock with table-driven
TestRunCheckLockSniff covering both trigger phrases and a benign
"locked-file" line that must not set LockPresent. Add
TestRunStatsZeroSnapshots to pin that RunStats accepts zero-snapshot
JSON without error.

internal/restic/runner.go

@@ -367,7 +367,7 @@ func (e Env) RunCheck(ctx context.Context, subsetPct int, handle LineHandler) (C
 	var res CheckResult
 	sniff := func(stream, line string, ev any) {
 		if stream == "stderr" {
-			if strings.Contains(line, "Found stale lock") || strings.Contains(line, "locked") {
+			if strings.Contains(line, "stale lock") || strings.Contains(line, "already locked") {
 				res.LockPresent = true
 			}
 		}