fix: BitmapFontManager getFont emitting TextStyle update event

[subhu339]

Description of change

Fixes an issue where BitmapFontManager.getFont() could emit a TextStyle update event as a side effect.
Instead of assigning lineHeight via the setter on a prototype-based style clone, the value is now defined directly on the cloned style instance to avoid triggering unintended updates.

Pre-Merge Checklist

Tests and/or benchmarks are included (behavior verified via manual reproduction)
Documentation is changed or added (not required for this internal fix)
Lint process passed (npm run lint)
Tests passed (npm run test)

Fixes #11723

[pkg-pr-new]

• pixi.js-base

• pixi.js-bunny-mark

  npm i https://pkg.pr.new/pixijs/pixijs/pixi.js@11835
  

commit: df43dd6

[Zyie]

hey @subhu339 can we get some tests added for this to verify we no longer emit extra events

[subhu339]

Added unit tests to verify that BitmapFontManager.getFont no longer emits a TextStyle update event.
Also updated the implementation to set _lineHeight directly on the cloned style instance, avoiding the setter and preventing unintended updates.