feat: add overflow 'fit' property for text labels

[utkarshhxd]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Adds a comprehensive visual test page for the proposed overflow: 'fit' text label behavior.

Fixed issues

• Label Text Width Fit Feature Request #21607

Details

Before: What was the problem?

There was no visual test coverage for the proposed overflow: 'fit' label behavior across different ECharts rendering scenarios.

After: How does it behave after the fixing?

This PR adds test/label-overflow-fit.html to demonstrate and validate:

• constrained-width label fitting
• plain text rendering
• rich text rendering
• markPoint label fitting
• Canvas vs SVG renderer behavior
• dynamic resizing behavior
• zero-width edge cases

Document Info

• This PR doesn't relate to document changes
• The document should be updated later
• The document changes have been made in apache/echarts-doc#xxx

Misc

Security Checking

• This PR uses security-sensitive Web APIs.

ZRender Changes

• This PR depends on ZRender changes: https://github.com/apache/zrender/pull/1157

Related test cases or examples to use the new APIs

Added test/label-overflow-fit.html.

Merging options

• Please squash the commits into a single one when merging.

[echarts-bot]

Thanks for your contribution!
The community will review it ASAP. In the meanwhile, please checkout the coding standard and Wiki about How to make a pull request.

Please DO NOT commit the files in dist, i18n, and ssr/client/dist folders in a non-release pull request. These folders are for release use only.

Document changes are required in this PR. Please also make a PR to apache/echarts-doc for document changes and update the issue id in the PR description. When the doc PR is merged, the maintainers will remove the PR: awaiting doc label.

This PR depends on ZRender changes. Please update the ZRender dependency to the latest nightly version including this change, which takes place everyday at 8:00 UTC (16:00 Beijing Time).
You can use npm i zrender@npm:zrender-nightly@dev to update package.json.
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