🧹 refactor FloatingActionAsyncButton build method and variants

[esenmx]

🎯 What: Refactored FloatingActionAsyncButton.build in lib/src/buttons/floating_action_async_button.dart to split the monolithic build method into cleaner helper methods (_buildButton, _buildStandard, _buildSmall, _buildLarge, and _buildExtended) and a _resolvedHeroTag getter.

💡 Why: Reduces the complexity and length of the build method from over 100 lines down to a concise dispatching method, improving readability, maintainability, and code health while reusing member state without argument repetition.

✅ Verification: Verified with dart format, flutter analyze (0 issues), and flutter test (70/70 passing tests).

✨ Result: Enhanced readability and maintainability of FloatingActionAsyncButton without changing any behavior or API contract.

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PR created automatically by Jules for task 18428285029922423034 started by @esenmx

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