fix: borderRadius incorrectly drawn in headings

[war-in]

Explanation of Change

Follow-up fix to #84556 (nested <Text> borderRadius). On iOS, a mention's rounded background inside a heading rendered wider than the text, bleeding past it.

Cause: the iOS path (RCTTextLayoutManagerWithBorderRadius) took the rect width from boundingRectForGlyphRange, whose trailing-whitespace/advance slack scales with font size. Fix: derive the horizontal extent from caret positions (locationForGlyphAtIndex), mirroring Android.

Fixed Issues

$ #78873
PROPOSAL:

Tests

1. On iOS: Native, open a chat composer and send a heading containing a mention, e.g. # abcd @here abcd.
2. Verify that the rounded background behind @here hugs the mention text and does not extend past it into the space before the next word.
3. Send a regular (non-heading) mention, e.g. Hello @here, and a heading long enough that a mention wraps to a second line; verify that backgrounds stay tight around the text and multiline spans round only the outer corners (left on the first line, right on the last).

Offline tests

N/A — mention rendering is local and does not depend on network state.

QA Steps

Same as tests.

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[github-actions]

⚠️ This PR is possibly changing native code and/or updating libraries, it may cause problems with HybridApp. Please check if any patch updates are required in the HybridApp repo and run an AdHoc build to verify that HybridApp will not break. Ask Contributor Plus for help if you are not sure how to handle this. ⚠️

[melvin-bot]

@parasharrajat Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

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[war-in]

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[parasharrajat]

Thanks for waiting. I had to upgrade mac and xcode for local ios build. I do not have access to Adhoc at the moment.

I will be testing it shortly