🧹 Refactor repetitive builder logic in Material button wrappers

[esenmx]

🎯 What: Consolidated repetitive AsyncButton builder logic and long-press gating in AsyncStandardMaterialButton, delegating specific Material widget creation to buildButton and buildIconButton in subclasses.
💡 Why: Reduces duplicate boilerplate across ElevatedAsyncButton, FilledAsyncButton, OutlinedAsyncButton, and TextAsyncButton, improving maintainability and code readability.
✅ Verification: Verified via flutter analyze and flutter test. All tests passed with zero linting/analysis errors.
✨ Result: Clean abstract builder pattern in AsyncStandardMaterialButton without changing any runtime behavior.

---

PR created automatically by Jules for task 1611094446663172648 started by @esenmx

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.