[ZEPPELIN-6516] Add visualization rendering E2E coverage

[miinhho]

What is this PR for?

Ths PR adds Playwright E2E coverage for Zeppelin's table and G2-based chart visualizations.

The test creates deterministic '%table' output through a notebook paragraph and verifies:

• Exact table headers and cell values
• Bar chart rendering
• Pie chart rendering
• Line chart rendering
• Area chart rendering
• Scatter chart rendering
• Table data preservation after switching between visualization modes

Since G2 renders charts on canvas, checking only that a canvas element is visible is not sufficient. The test also inspects the canvas alpha channel and verifies that it contains rendered pixels.

A dedicated NotebookVisualizationPage Page Object is added to isolate visualization selectors, table locators, chart canvas locators, and canvas-rendering inspection from the scenario code.

Todos

• Add E2E coverage for table rendering
• Add E2E coverage for bar chart rendering
• Add E2E coverage for pie chart rendering
• Add E2E coverage for line chart rendering
• Add E2E coverage for area chart rendering
• Add E2E coverage for scatter chart rendering

What type of PR is it?

Improvement

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-6516

How should this be tested?

Start the Zeppelin backend, then run the focused Chromium E2E test:

cd zeppelin-web-angular
npm run e2e:fast -- tests/notebook/paragraph/visualization-rendering.spec.ts

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[voidmatcha]

Passing only the table test also counts all five charts as successful in the coverage report. I've put together an example fix that adds each chart's annotation when its test step is reached. Please take a look.

[voidmatcha]

LGTM 👍

[voidmatcha]

Merged into master (ec96819).