fix(#275): Prevent silent connection initiation failures

Fixed silent connection initiation failures where HTTP errors were caught
but success was returned to the user, leaving zombie connections in
PENDING state forever.

Changes:
- Delete failed connection from database when HTTP request fails
- Throw BadRequestException with clear error message
- Added test to verify connection deletion and exception throwing
- Import BadRequestException in connection.service.ts

User Impact:
- Users now receive immediate feedback when connection initiation fails
- No more zombie connections stuck in PENDING state
- Clear error messages indicate the reason for failure

Testing:
- Added test case: "should delete connection and throw error if request fails"
- All 21 connection service tests passing
- Quality gates: lint, typecheck, build all passing

Fixes #275

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

apps/api/src/federation/connection.service.spec.ts

@@ -85,6 +85,7 @@ describe("ConnectionService", () => {
               findUnique: vi.fn(),
               findMany: vi.fn(),
               update: vi.fn(),
+              delete: vi.fn(),
             },
           },
         },
@@ -208,6 +209,36 @@ describe("ConnectionService", () => {
         })
       );
     });
+
+    it("should delete connection and throw error if request fails", async () => {
+      const mockAxiosResponse: AxiosResponse = {
+        data: mockRemoteIdentity,
+        status: 200,
+        statusText: "OK",
+        headers: {},
+        config: {} as never,
+      };
+
+      vi.spyOn(httpService, "get").mockReturnValue(of(mockAxiosResponse));
+      vi.spyOn(httpService, "post").mockReturnValue(
+        throwError(() => new Error("Connection refused"))
+      );
+      const createSpy = vi
+        .spyOn(prismaService.federationConnection, "create")
+        .mockResolvedValue(mockConnection);
+      const deleteSpy = vi
+        .spyOn(prismaService.federationConnection, "delete")
+        .mockResolvedValue(mockConnection);
+
+      await expect(service.initiateConnection(mockWorkspaceId, mockRemoteUrl)).rejects.toThrow(
+        "Failed to initiate connection"
+      );
+
+      expect(createSpy).toHaveBeenCalled();
+      expect(deleteSpy).toHaveBeenCalledWith({
+        where: { id: mockConnection.id },
+      });
+    });
   });
 
   describe("acceptConnection", () => {