fix(heatmap): add eventData handler for proper click event data

[ubertidavide]

Description

When interacting with heatmaps, the standard plotly_click event previously lacked properly formatted 2D matrix indices and explicit z values. This made it difficult for downstream wrappers (such as Streamlit or Marimo) to identify the exact clicked cell natively.

Architecture Note: plotly_click vs plotly_heatmapclick

While introducing a trace-specific event like plotly_heatmapclick (similar to plotly_treemapclick or plotly_sunburstclick) was originally discussed in #7685, doing so for heatmaps is an anti-pattern. Heatmaps are Cartesian trace types that reside on standard 2D Cartesian subplots. Introducing trace-specific click events on Cartesian subplots breaks consistency across Cartesian traces. The correct architectural approach is to rely on the standard plotly_click event and delegate trace-specific payload formatting to a dedicated eventData handler.

Summary of Changes

This PR introduces the missing eventData handler for the heatmap trace:

• pointNumber and pointIndex are properly exposed as a 2D array [row, col].
• z values are explicitly attached to the event payload.

This PR completes and supersedes #7686 (which was stalled). Unlike #7686, this implementation cleanly exposes the 2D pointNumber: [row, col] matrix indices and z values via eventData without modifying hover.js (avoiding category axis regressions), and includes complete Jasmine unit tests.

Closes #7685.

Testing

• Added regression tests in test/jasmine/tests/heatmap_test.js to verify that standard plotly_click events emit 2D pointNumber and z values.
• Ran Jasmine test suite (npm run test-jasmine -- test/jasmine/tests/heatmap_test.js) with 51/51 specs passing successfully.