Fixed empty text of model selection button when no model is chosen

[Aman071106]

PR Description

Fixed Issue #942: Empty text of Select model button when no model is chosen.

1. Modified AIModelSelectorButton to display 'Select Model' when the model name is null or empty.
2. Added unit tests in test/screens/common_widgets/ai/ai_model_selector_button_test.dart to verify the behavior for null, empty, and valid model states.

Related Issues

• Closes Empty text of Select model button when no model is chosen. #942

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux

Reference video

fix.issue.942.mp4