fix: handle error when jwt token could not be retrieved

[Ruadhri17]

Proposed changes

This PR replace unwrap() with anyhow error handling when c8y_auth_proxy is trying to retrive JWT token.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

• Unchecked unwrap resulting in a panic in JWT handling #3019

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s)
• I ran cargo fmt as mentioned in CODING_GUIDELINES
• I used cargo clippy as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/extensions/c8y_auth_proxy/src/server.rs	68.00%	4 Missing and 4 partials ⚠️
crates/extensions/c8y_auth_proxy/src/tokens.rs	71.42%	0 Missing and 2 partials ⚠️

Additional details and impacted files

📢 Thoughts on this report? Let us know!

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
504	0	2	504	100	1h24m33.94476s

[didier-wenzek]

Approved

[rina23q]

LGTM