fix(tests): Correct pipeline test failures (#239)

Fixes 4 test failures identified in pipeline run 239:

1. RunnerJobsService cancel tests:
   - Use updateMany mock instead of update (service uses optimistic locking)
   - Add version field to mock objects
   - Use mockResolvedValueOnce for sequential findUnique calls

2. ActivityService error handling tests:
   - Update tests to expect null return (fire-and-forget pattern)
   - Activity logging now returns null on DB errors per security fix

3. SecretScannerService unreadable file test:
   - Handle root user case where chmod 0o000 doesn't prevent reads
   - Test now adapts expectations based on runtime permissions

Quality gates: lint ✓ typecheck ✓ tests ✓
- @mosaic/orchestrator: 612 tests passing
- @mosaic/web: 650 tests passing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/activity/activity.service.spec.ts

@@ -802,7 +802,7 @@ describe("ActivityService", () => {
       );
     });
 
-    it("should handle database errors gracefully when logging activity", async () => {
+    it("should handle database errors gracefully when logging activity (fire-and-forget)", async () => {
       const input: CreateActivityLogInput = {
         workspaceId: "workspace-123",
         userId: "user-123",
@@ -814,7 +814,9 @@ describe("ActivityService", () => {
       const dbError = new Error("Database connection failed");
       mockPrismaService.activityLog.create.mockRejectedValue(dbError);
 
-      await expect(service.logActivity(input)).rejects.toThrow("Database connection failed");
+      // Activity logging is fire-and-forget - returns null on error instead of throwing
+      const result = await service.logActivity(input);
+      expect(result).toBeNull();
     });
 
     it("should handle extremely large details objects", async () => {
@@ -1132,7 +1134,7 @@ describe("ActivityService", () => {
   });
 
   describe("database error handling", () => {
-    it("should handle database connection failures in logActivity", async () => {
+    it("should handle database connection failures in logActivity (fire-and-forget)", async () => {
       const createInput: CreateActivityLogInput = {
         workspaceId: "workspace-123",
         userId: "user-123",
@@ -1144,7 +1146,9 @@ describe("ActivityService", () => {
       const dbError = new Error("Connection refused");
       mockPrismaService.activityLog.create.mockRejectedValue(dbError);
 
-      await expect(service.logActivity(createInput)).rejects.toThrow("Connection refused");
+      // Activity logging is fire-and-forget - returns null on error instead of throwing
+      const result = await service.logActivity(createInput);
+      expect(result).toBeNull();
     });
 
     it("should handle Prisma timeout errors in findAll", async () => {