[google_fonts] Remove failed loads from pendingFonts

[Yusufihsangorgel]

Failed font loads remain in pendingFontFutures because the current cleanup
callback runs only after successful completion. As a result, every later
GoogleFonts.pendingFonts() call rethrows the same completed error.

This change removes a tracked load on either completion path while preserving
the original error for the caller currently awaiting pendingFonts(). It adds
regression coverage showing that the first call reports the load failure and a
later call is no longer poisoned by it.

Fixes flutter/flutter#182430.

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[gemini-code-assist]

Code Review

This pull request updates the google_fonts package to version 8.2.1. It ensures that failed font loads are removed from pendingFontFutures by replacing then with whenComplete on the loading future in google_fonts_base.dart. A unit test has been added to verify that pendingFonts correctly removes failed font loads. There are no review comments, and I have no feedback to provide.