fix: add stricter validation for Ed25519 Signed Payload

[overcat]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.02%. Comparing base (2ff38ac) to head (95112af).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #751      +/-   ##
============================================
+ Coverage     80.96%   81.02%   +0.05%     
- Complexity     1386     1397      +11     
============================================
  Files           216      216              
  Lines          5349     5365      +16     
  Branches        518      526       +8     
============================================
+ Hits           4331     4347      +16     
  Misses          715      715              
  Partials        303      303              

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/SignerKey.java	91.11% <100.00%> (+1.23%)	⬆️
src/main/java/org/stellar/sdk/StrKey.java	91.85% <100.00%> (+0.15%)	⬆️

🚀 New features to boost your workflow:

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