fix: Rejecting malformed ECDSA signatures

[dadrus]

This PR hardens signature verification against malformed, attacker-controlled signature input.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (25f319d) to head (73bcb69).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   94.52%   94.53%   +0.01%     
==========================================
  Files          18       18              
  Lines        1314     1317       +3     
==========================================
+ Hits         1242     1245       +3     
  Misses         37       37              
  Partials       35       35              

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