[Feature] add shouldOpen callback to conditionally allow ExpandableFab opening

[GeekTree0101]

Summary

This PR introduces a new optional callback shouldOpen to the ExpandableFab widget, allowing developers to conditionally block the FAB menu from opening.

Changes

• Added final bool Function()? shouldOpen to ExpandableFab
• Updated toggle() and open() methods to respect shouldOpen result
• If shouldOpen returns false, the menu will not open
• Ensured backward compatibility: if shouldOpen is null, menu opens as usual

Motivation

This provides better control for developers who want to prevent opening the FAB under certain conditions (e.g., unsaved changes, dialogs open, etc).

Example

ExpandableFab(
  shouldOpen: () => !_isLocked,
  ...
)