feat: implement message signing and verification according to SEP-53.

[overcat]

No description provided.

[Copilot]

Pull Request Overview

This PR adds end-to-end message signing and verification methods to KeyPair following SEP-53, along with unit tests and a changelog entry.

• Introduce calculateMessageHash, signMessage, and verifyMessage methods in KeyPair
• Add KeyPairTest cases for valid and invalid message signatures
• Update CHANGELOG.md with the new feature entry

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/org/stellar/sdk/KeyPair.java	Implement SEP-53 hash, sign, and verify methods
src/test/java/org/stellar/sdk/KeyPairTest.java	Add tests for signing/verifying valid and invalid messages
CHANGELOG.md	Document new message signing/verification feature

Comments suppressed due to low confidence (2)

src/main/java/org/stellar/sdk/KeyPair.java:359

• [nitpick] The Javadoc anchor includes target="_blank", which isn't officially supported in standard Javadoc rendering. Consider removing or escaping the target attribute to avoid warnings or broken links in generated docs.

   * Calculate the hash of a message according to <a

src/test/java/org/stellar/sdk/KeyPairTest.java:246

• There's no test case for signing/verifying an empty message. Adding a test for an empty byte array or empty string would ensure edge-case coverage for the new methods.

  @Test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.25%. Comparing base (2989e9b) to head (09dfab8).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #698      +/-   ##
============================================
+ Coverage     80.21%   80.25%   +0.04%     
- Complexity     1232     1237       +5     
============================================
  Files           213      213              
  Lines          4867     4878      +11     
  Branches        415      415              
============================================
+ Hits           3904     3915      +11     
  Misses          707      707              
  Partials        256      256              

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/KeyPair.java	85.47% <100.00%> (+1.50%)	⬆️

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