Improved mutex poisoned error messages in examples

[yashithac618]

Description

This PR replaces unwrap() calls on Mutex::lock() with expect("mutex poisoned") in the example applications.
This provides clearer error messages in the event of mutex poisoning and improves debugging experience.
Changes are limited to the examples/ directory. No core library logic or public API is modified.

Notes to the reviewers

This change is intentionally limited to example applications only.
Core crates were not modified.

The behavior remains the same — only the error message in the unlikely event of mutex poisoning is improved.

Changelog notice

None (examples-only change).

Checklists

All Submissions:

• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR