refactor: Delete SecretKey compare op from library and move it to tests module

[pratikmankawde]

High Level Overview of Change

Delete the SecretKey equality/inequality operators from the public library header and move the comparison logic into test-only code.

The operator== and operator!= free functions on SecretKey were removed from include/xrpl/protocol/SecretKey.h and replaced with explicitly deleted member functions to prevent accidental use in production code. A named test::equal() helper was added in src/test/unit_test/utils.h for test assertions that need to compare secret keys.

Context of Change

Comparing secret keys with == in production code is a security concern — constant-time comparison should be used instead. By deleting the operator and restricting comparison to test code only, we prevent accidental timing-side-channel vulnerabilities in the library.

The previous free-function operator==/operator!= overloads in SecretKey.h were available to all consumers of libxrpl. Now, the deleted member operators cause a compile error if anyone tries to compare SecretKey objects outside of test code. Test files that need comparison use the explicit test::equal() named function, which bypasses the deleted-member-function lookup issue that would affect any free-function operator== overload.

Type of Change

• Refactor (non-breaking change that only restructures code)

API Impact

• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)

Before / After

Before: SecretKey had public operator== / operator!= free functions in the library header, usable by any code that included SecretKey.h.

// Production or test code — both compiled fine
BEAST_EXPECT(k.keys->secretKey == tokenSecretKey);

After: SecretKey::operator== is deleted. Production code gets a compile error. Test code uses the named helper:

// Production code — compile error (good)
// k.keys->secretKey == tokenSecretKey;

// Test code — explicit opt-in
BEAST_EXPECT(test::equal(k.keys->secretKey, tokenSecretKey));

Test Plan

• Build passes with zero errors
• All existing SecretKey comparison test assertions updated to use test::equal()
• Affected test files: SecretKey_test.cpp, ValidatorKeys_test.cpp, Manifest_test.cpp

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.8%. Comparing base (1bf9e6e) to head (170c613).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6503     +/-   ##
=========================================
- Coverage     79.8%   79.8%   -0.0%     
=========================================
  Files          878     878             
  Lines        67887   67885      -2     
  Branches      7551    7553      +2     
=========================================
- Hits         54206   54195     -11     
- Misses       13681   13690      +9     

Files with missing lines	Coverage Δ
include/xrpl/protocol/SecretKey.h	66.7% <ø> (-4.8%)	⬇️

... and 6 files with indirect coverage changes

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