(ci) Refactor alpha update E2E tests and increase coverage

- Refactored the alpha update test suite for clarity and modularity.
- Added validation for preserved custom code and go.mod dependencies.
- Improved version-specific assertions to avoid regressions.
- Increased e2e test coverage for v4.6.0 and v4.7.0 update paths.

Assisted-by: ChatGPT (OpenAI) and GitHub Copilot

Author: camilamacedo86

.gitignore

@@ -28,3 +28,4 @@ docs/book/src/docs
 /testdata/**/go.sum
 /docs/book/src/simple-external-plugin-tutorial/testdata/sampleexternalplugin/v1/bin
 /testdata/**legacy**
+/test/e2e/alphaupdate/e2e-**