Update ResolvedStyledText API

[Kyle-Ye]

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🤖 Augment PR Summary

Summary: This PR updates the text rendering/measuring surface area, primarily by implementing CoreText-backed font-metrics aggregation for attributed strings and refining ResolvedStyledText construction/behavior.

Changes:

• Implemented `NSAttributedString.maxFontMetrics` (CoreText path) and added a focused test suite for it.
• Added CoreText private API shims (language-aware outsets, clipping metrics, excessive-line-height character set) and utilities to compute oversized-character drawing margins.
• Hooked `Text.Sizing.layoutMargins(...)` up to the new metrics/margin logic, including vertical writing-mode margin rotation and modifier adjustments.
• Refactored `AttributeCountInfoKey` into its own file to keep test utilities separated.
• Reworked parts of `ResolvedStyledText`: added caching for max font metrics, implemented `isEmpty`, dynamic resolution helpers, and introduced an explicit `styledText(...)` factory choosing between nested `StringDrawing` vs `TextLayoutManager` implementations.
• Added a non-Darwin shim for `NSStringDrawingContext` and a non-Darwin workaround initializer for `NSAttributedString` (Swift issue 71874).
• Made `NSAttributedString.updateSchedule` return a safe default when missing.
• Introduced a new WIP `Text+Measure` scaffold (metrics types + cache placeholders) for future measurement work.

Technical Notes: Many code paths are gated behind canImport(CoreText) with explicit fallbacks on other platforms; several text rendering pieces remain marked WIP/FIXME (notably the measurement pipeline and ORB-backed rendering).

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Codecov Report

❌ Patch coverage is 0% with 276 lines in your changes missing coverage. Please review.
✅ Project coverage is 14.20%. Comparing base (5551939) to head (666b529).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ces/OpenSwiftUICore/View/Text/Text/Text+View.swift	0.00%	189 Missing ⚠️
...s/OpenSwiftUICore/View/Text/Text/Text+Sizing.swift	0.00%	28 Missing ⚠️
.../OpenSwiftUICore/View/Text/Text/Text+Measure.swift	0.00%	27 Missing ⚠️
...ext/AttributedString/Text+NSAttributedString.swift	0.00%	14 Missing ⚠️
...ICore/View/Text/Util/ReducedTimelineSchedule.swift	0.00%	9 Missing ⚠️
...rces/OpenSwiftUICore/Shim/TextNonDarwinShims.swift	0.00%	7 Missing ⚠️
...s/OpenSwiftUICore/Test/AttributeCountInfoKey.swift	0.00%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (5551939) and HEAD (666b529). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (5551939)	HEAD (666b529)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #945       +/-   ##
===========================================
- Coverage   26.28%   14.20%   -12.08%     
===========================================
  Files         724      683       -41     
  Lines       51368    45197     -6171     
===========================================
- Hits        13500     6422     -7078     
- Misses      37868    38775      +907     

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