fix(text): improve overflow fit behavior

[utkarshhxd]

Summary

Add support for overflow: 'fit' text behavior in ZRender.

This change implements horizontal text scaling to fit constrained widths and adds visual test coverage for:

• normal fit overflow scaling
• zero-width edge cases

Changes

• Added overflow: 'fit' scaling behavior for plain text
• Handled edge cases when width is zero
• Added visual test cases in test/text.html

Testing

Verified visually through test/text.html:

• text scales correctly within constrained width
• zero-width case does not crash or produce invalid rendering
• no console errors observed

Related to apache/echarts#21607.

This PR implements the underlying text scaling behavior in ZRender required for supporting overflow: 'fit'.