refactor!: return signature SCVal from Auth.Signer for custom account contracts

[overcat]

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[Copilot]

Pull request overview

This PR refactors Soroban authorization signing to support custom account contracts by changing Auth.Signer to return a signature SCVal directly (attached verbatim), and adds helper APIs to build the default Stellar Account signature shape and payload hash.

Changes:

• Breaking API change: Auth.Signer.sign(HashIDPreimage) now returns SCVal (and Auth.Signature / client-side ed25519 verification are removed).
• Added Auth.authorizationPayloadHash(...) and Auth.defaultAccountSignatureScVal(...) helpers; updated authorizeInvocation(..., publicKey) parameter naming to address.
• Centralized lexicographic unsigned byte comparison into Util.compareBytesUnsigned(...) and updated comparators/sorting logic; added/updated tests and changelog notes.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/test/kotlin/org/stellar/sdk/UtilTest.kt	Adds unit tests for Util.compareBytesUnsigned.
src/test/java/org/stellar/sdk/AuthTest.java	Updates signing tests for new Signer contract; adds coverage for custom signature SCVal behavior and helper methods.
src/main/java/org/stellar/sdk/Util.java	Adds compareBytesUnsigned(byte[], byte[]) utility.
src/main/java/org/stellar/sdk/scval/ScvComparator.java	Switches byte comparisons to Util.compareBytesUnsigned for SCVal/address ordering.
src/main/java/org/stellar/sdk/Auth.java	Implements the Signer -> SCVal refactor; adds helper methods; updates delegate sorting to use shared unsigned byte comparator.
CHANGELOG.md	Documents the breaking change and migration snippet for Auth.Signer.

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[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.91%. Comparing base (d9632c9) to head (7e5dcd0).

Files with missing lines	Patch %	Lines
src/main/java/org/stellar/sdk/Auth.java	90.90%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #806      +/-   ##
============================================
- Coverage     82.92%   82.91%   -0.01%     
  Complexity     1660     1660              
============================================
  Files           232      232              
  Lines          6060     6046      -14     
  Branches        678      677       -1     
============================================
- Hits           5025     5013      -12     
+ Misses          658      656       -2     
  Partials        377      377              

Files with missing lines	Coverage Δ
src/main/java/org/stellar/sdk/Util.java	80.00% <100.00%> (+2.22%)	⬆️
...main/java/org/stellar/sdk/scval/ScvComparator.java	93.52% <100.00%> (-0.19%)	⬇️
src/main/java/org/stellar/sdk/Auth.java	86.41% <90.90%> (+0.98%)	⬆️

... and 1 file with indirect coverage changes

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