feat: better handling of resend requests

[davidsteiner]

This change contains the following improvements for resend requests:

• correct validation and rejection of messages with invalid fields
• skipping the resend logic altogether when there is no established connection

[codecov]

Codecov Report

❌ Patch coverage is 88.63636% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.55%. Comparing base (f7c0ddb) to head (8e888d5).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/hotfix/src/session.rs	88.37%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   83.25%   84.55%   +1.29%     
==========================================
  Files          75       75              
  Lines        6672     6702      +30     
==========================================
+ Hits         5555     5667     +112     
+ Misses       1117     1035      -82     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.