test: fix unit tests for user_agent and proxy repo interface

- Add user_agent field to API contract test expectation
- Add ListWithFiltersAndAccountCount stub to proxyRepoStub

backend/internal/server/api_contract_test.go

@@ -243,7 +243,8 @@ func TestAPIContracts(t *testing.T) {
 							"first_token_ms": 50,
 							"image_count": 0,
 							"image_size": null,
-							"created_at": "2025-01-02T03:04:05Z"
+							"created_at": "2025-01-02T03:04:05Z",
+							"user_agent": null
 						}
 					],
 					"total": 1,

backend/internal/service/admin_service_delete_test.go

@@ -186,6 +186,10 @@ func (s *proxyRepoStub) ListActiveWithAccountCount(ctx context.Context) ([]Proxy
 	panic("unexpected ListActiveWithAccountCount call")
 }
 
+func (s *proxyRepoStub) ListWithFiltersAndAccountCount(ctx context.Context, params pagination.PaginationParams, protocol, status, search string) ([]ProxyWithAccountCount, *pagination.PaginationResult, error) {
+	panic("unexpected ListWithFiltersAndAccountCount call")
+}
+
 func (s *proxyRepoStub) ExistsByHostPortAuth(ctx context.Context, host string, port int, username, password string) (bool, error) {
 	panic("unexpected ExistsByHostPortAuth call")
 }