docs: add widget testing example

[xianjianlf2]

Summary

• Add a README example for widget testing OfflineBuilder.
• Show how to inject fake Connectivity and NetworkInfo services with OfflineBuilder.initialize.
• Document that tests should pump past the debounce duration before using finders.

Related to #41.

Validation

• Not run locally: dart and flutter are not installed in this environment.
• Verified the change is README-only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8062f54) to head (ce15e08).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #73   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           86        86           
=========================================
  Hits            86        86           

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