fix: enhance useImport tests with improved structure and error handling

- Introduced a new wrapper function for query client to facilitate testing.
- Added comprehensive tests for upload, commit, and cancel operations.
- Improved error handling in tests to capture and assert error states.
- Enhanced session management and state reset functionality in tests.
- Implemented polling behavior tests for import status and preview queries.
- Ensured that upload previews are prioritized over status query previews.
- Validated cache invalidation and state management after commit and cancel actions.

backend/internal/api/handlers/testdata/fake_caddy_conflict.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+if [ "$1" = "version" ]; then
+  echo "v2.0.0"
+  exit 0
+fi
+if [ "$1" = "fmt" ]; then
+  exit 0
+fi
+if [ "$1" = "adapt" ]; then
+  # Return a host that conflicts with existing (conflict.example.com)
+  echo "{\"apps\":{\"http\":{\"servers\":{\"srv0\":{\"routes\":[{\"match\":[{\"host\":[\"conflict.example.com\"]}],\"handle\":[{\"handler\":\"reverse_proxy\",\"upstreams\":[{\"dial\":\"192.168.1.100:9000\"}]}]}]}}}}}"
+  exit 0
+fi
+exit 1