fix: app‑extension‑safe guard and fallback in RCTFontSizeMultiplier()

[SmartArray]

Summary:

This change fixes a text rendering issue on iOS when using the new architecture (Fabric), most notably in app extensions on iOS.

While migrating an app extension to the new architecture, all the labels in our app became invisible. A bare <Text> component initially appeared to work, which made the issue hard to track down.

Text rendered correctly with default styles, but once fontSize or fontFamily was applied, text would ignore sizing or become invisible. Because text was still visible by default, the problem did not immediately appear to be related to font scaling or measurement, so it took me WEEKS to understand.
Please check out the issue I created almost a month ago: #54642

After deeper investigation into Fabric text measurement and after using 100 different breakpoint attempts, the root cause turned out to be an invalid font size multiplier coming from RCTFontSizeMultiplier() when running inside an app extension.

In app extensions, RCTSharedApplication() can (or will!) return nil. When that happened, RCTFontSizeMultiplier() could return an invalid value, which later caused Fabric text measurement to produce zero-height paragraphs. As a result, <Text> components could ignore fontSize or become invisible when fontFamily was set.

The fix makes RCTFontSizeMultiplier() app-extension-safe by returning a positive default (1.0) when UIApplication or the preferred content size category is unavailable, and by clamping invalid values.

Related issue:
#54642

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Changelog:

[IOS] [FIXED] - Make RCTFontSizeMultiplier() app-extension-safe by adding a default fallback and clamping invalid values to a positive number, such that fonts are never scaled down to zero

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Test Plan:

• Reproduced the issue in an iOS app extension using Fabric
• Observed <Text> rendering correctly by default but ignoring fontSize or becoming invisible when fontFamily was applied
• Traced the issue through Fabric text measurement and font size multiplier handling
• Applied the fix to RCTFontSizeMultiplier()
• Verified that text renders correctly and respects fontSize
• Confirmed no behavior change in a regular iOS app

To test this fix the repro that I provided in the issue above can easily be used.
First run the code without the commit (observe the bad outcome) then run it with the commit (observe the good outcome)

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