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
 			}
 		}

internal/restic/runner_test.go

@@ -54,18 +54,34 @@ func TestRunPruneInvokesPrune(t *testing.T) {
 
 // --- B2: RunCheck ---
 
-func TestRunCheckParsesLock(t *testing.T) {
-	bin := setupScriptBin(t, `echo "Found stale lock" >&2`)
-	env := Env{Bin: bin}
-	res, err := env.RunCheck(context.Background(), 0, nil)
-	if err != nil {
-		t.Fatalf("RunCheck returned unexpected error: %v", err)
+func TestRunCheckLockSniff(t *testing.T) {
+	cases := []struct {
+		name       string
+		stderrLine string
+		wantLocked bool
+	}{
+		{"stale lock", "Found stale lock from PID 1234", true},
+		{"already locked", "repository is already locked exclusively", true},
+		{"benign mention", "subdir/locked-file ok", false},
+		{"empty", "", false},
 	}
-	if !res.LockPresent {
-		t.Fatal("expected LockPresent=true")
-	}
-	if res.ErrorsFound {
-		t.Fatal("expected ErrorsFound=false")
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			// Script emits the line on stderr, then exits 0.
+			script := fmt.Sprintf(`printf '%%s\n' %q >&2`, c.stderrLine)
+			bin := setupScriptBin(t, script)
+			env := Env{Bin: bin}
+			res, err := env.RunCheck(context.Background(), 0, nil)
+			if err != nil {
+				t.Fatalf("RunCheck returned unexpected error: %v", err)
+			}
+			if res.LockPresent != c.wantLocked {
+				t.Fatalf("LockPresent: got %v, want %v (line: %q)", res.LockPresent, c.wantLocked, c.stderrLine)
+			}
+			if res.ErrorsFound {
+				t.Fatal("expected ErrorsFound=false")
+			}
+		})
 	}
 }
 
@@ -149,3 +165,21 @@ func TestRunStatsErrorsWithoutJSON(t *testing.T) {
 		t.Fatalf("unexpected error: %v", err)
 	}
 }
+
+func TestRunStatsZeroSnapshots(t *testing.T) {
+	// Confirms RunStats succeeds and returns a valid *RepoStats when the
+	// repo has no snapshots (snapshots_count=0). A regression that
+	// re-added a "SnapshotsCount > 0" guard would return an error here.
+	bin := setupScriptBin(t, `echo '{"total_size":0,"total_uncompressed_size":0,"snapshots_count":0,"total_file_count":0,"total_blob_count":0}'`)
+	env := Env{Bin: bin}
+	stats, err := env.RunStats(context.Background(), nil)
+	if err != nil {
+		t.Fatalf("RunStats with zero snapshots returned unexpected error: %v", err)
+	}
+	if stats == nil {
+		t.Fatal("expected non-nil *RepoStats, got nil")
+	}
+	if stats.SnapshotsCount != 0 {
+		t.Fatalf("SnapshotsCount: got %d, want 0", stats.SnapshotsCount)
+	}
+}