Fix config validation edge cases

internal/core/diagnostics/run_dir_test.go

@@ -2,6 +2,7 @@ package diagnostics
 
 import (
 	"encoding/json"
+	"fmt"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -35,6 +36,57 @@ func TestNewRunDirectoryCreatesRunDirectoryAndRunID(t *testing.T) {
 	}
 }
 
+func TestNewRunDirectoryRetriesOnRunIDCollision(t *testing.T) {
+	workDir := t.TempDir()
+	first := time.Unix(0, 100).UTC()
+	second := first.Add(time.Nanosecond)
+	if err := os.Mkdir(filepath.Join(workDir, fmt.Sprintf("run-%d", first.UnixNano())), 0o755); err != nil {
+		t.Fatalf("create existing run directory: %v", err)
+	}
+	restoreUTCNow := replaceUTCNow(func() func() time.Time {
+		calls := 0
+		return func() time.Time {
+			calls++
+			if calls == 1 {
+				return first
+			}
+			return second
+		}
+	}())
+	t.Cleanup(restoreUTCNow)
+
+	runDir, err := NewRunDirectory(workDir, RetentionAuto)
+	if err != nil {
+		t.Fatalf("NewRunDirectory: %v", err)
+	}
+
+	wantRunID := fmt.Sprintf("run-%d", second.UnixNano())
+	if runDir.RunID() != wantRunID {
+		t.Fatalf("RunID = %q, want %q", runDir.RunID(), wantRunID)
+	}
+	if _, err := os.Stat(runDir.Path()); err != nil {
+		t.Fatalf("stat run directory: %v", err)
+	}
+}
+
+func TestNewRunDirectoryReturnsErrorAfterRunIDCollisionsExhausted(t *testing.T) {
+	workDir := t.TempDir()
+	collisionTime := time.Unix(0, 200).UTC()
+	collisionPath := filepath.Join(workDir, fmt.Sprintf("run-%d", collisionTime.UnixNano()))
+	if err := os.Mkdir(collisionPath, 0o755); err != nil {
+		t.Fatalf("create existing run directory: %v", err)
+	}
+	restoreUTCNow := replaceUTCNow(func() time.Time {
+		return collisionTime
+	})
+	t.Cleanup(restoreUTCNow)
+
+	_, err := NewRunDirectory(workDir, RetentionAuto)
+	if err == nil || !strings.Contains(err.Error(), "exhausted unique run ID attempts") {
+		t.Fatalf("expected exhausted collision error, got %v", err)
+	}
+}
+
 func TestNewRunDirectoryUsesDefaultWorkDirectory(t *testing.T) {
 	runDir, err := NewRunDirectory("", RetentionAuto)
 	if err != nil {
@@ -252,6 +304,14 @@ func newTestRunDirectory(t *testing.T) *RunDirectory {
 	return runDir
 }
 
+func replaceUTCNow(replacement func() time.Time) func() {
+	original := utcNow
+	utcNow = replacement
+	return func() {
+		utcNow = original
+	}
+}
+
 func readArtifact(t *testing.T, runDir *RunDirectory, name string) []byte {
 	t.Helper()
 	data, err := os.ReadFile(filepath.Join(runDir.Path(), name))