test: use more robust http parsing in the c8y-proxy host header tests

[jarhodes314]

Proposed changes

Improve the host header tests introduced in #2946 by actually parsing the headers instead of simply checking for the presence of a string in the incoming request. Additionally simplify the logic for obtaining the websocket host.

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

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 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.3%. Comparing base (e54f81a) to head (d1359b0).
Report is 1 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
crates/extensions/c8y_auth_proxy/src/server.rs	89.8% <93.3%> (+1.0%)	⬆️

... and 4 files with indirect coverage changes

[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
453	0	3	453	100	58m36.590424999s

[didier-wenzek]

Approved