fix(server): Address PR #733 review feedback and fix cross-platform tests

- Extract merge logic from pipeline-orchestrator to merge-service.ts to avoid HTTP self-call
- Make agent-executor error handling provider-agnostic using shared isAuthenticationError utility
- Fix cross-platform path handling in tests using path.normalize/path.resolve helpers
- Add catch handlers in plan-approval-service tests to prevent unhandled promise rejection warnings

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

apps/server/tests/unit/services/plan-approval-service.test.ts

@@ -54,6 +54,8 @@ describe('PlanApprovalService', () => {
 
     it('should timeout and reject after configured period', async () => {
       const approvalPromise = service.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       // Flush the async initialization
       await vi.advanceTimersByTimeAsync(0);
 
@@ -73,6 +75,8 @@ describe('PlanApprovalService', () => {
       } as never);
 
       const approvalPromise = service.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       // Flush the async initialization
       await vi.advanceTimersByTimeAsync(0);
 
@@ -93,6 +97,8 @@ describe('PlanApprovalService', () => {
       );
 
       const approvalPromise = serviceNoSettings.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       // Flush async
       await vi.advanceTimersByTimeAsync(0);
 
@@ -417,6 +423,8 @@ describe('PlanApprovalService', () => {
       } as never);
 
       const approvalPromise = service.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       await vi.advanceTimersByTimeAsync(0);
 
       // Should not timeout at 4 minutes
@@ -432,6 +440,8 @@ describe('PlanApprovalService', () => {
       vi.mocked(mockSettingsService!.getProjectSettings).mockRejectedValue(new Error('Failed'));
 
       const approvalPromise = service.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       await vi.advanceTimersByTimeAsync(0);
 
       // Should use default 30 minute timeout
@@ -448,6 +458,8 @@ describe('PlanApprovalService', () => {
       } as never);
 
       const approvalPromise = service.waitForApproval('feature-1', '/project');
+      // Attach catch to prevent unhandled rejection warning (will be properly asserted below)
+      approvalPromise.catch(() => {});
       await vi.advanceTimersByTimeAsync(0);
 
       // Should use default 30 minute timeout