Fix handling of special characters in categorical values

[kaituo]

Description

This PR addresses a bug that occurred when categorical values contained special characters such as colons. Previously, entities were stored as a comma-separated name-value pair string in the heatmap cell's custom data field. When a cell was clicked, the custom data was retrieved and the string was split via colons. This caused errors if the categorical value itself contained colons or if multiple name-value pairs included commas.

To fix this issue, this PR changes the storage format to a JSON string in the cell's custom data. When a cell is clicked, the JSON object is restored. Since JSON naturally handles special characters, this eliminates the need for additional special character handling.

Additionally, the previous implementation stored a newline-separated name-value pair in the y-axis and a comma-separated name-value pair in the cell tooltip. With entities now stored as JSON in custom data, we no longer have the flexibility to store these different formats. This PR standardizes the format to newline-separated name-value pairs for both the y-axis and cell tooltip.

Testing Done:

• Conducted end-to-end testing in preview, historical, and real-time use cases.
• Added unit tests.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.05%. Comparing base (e5cf2d8) to head (c2780a4).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #757      +/-   ##
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- Coverage   50.31%   50.05%   -0.27%     
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  Files         166      166              
  Lines        5595     5922     +327     
  Branches     1075     1186     +111     
==========================================
+ Hits         2815     2964     +149     
- Misses       2508     2644     +136     
- Partials      272      314      +42     

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[kaituo]

Not sure why coverage CI failed as I only added tests and didn't delete tests. Will leave it as it is.

[jackiehanyang]

Thanks for the fix and adding tests. LGTM!