fix: add spring-core test dependency for spring-security-crypto 7.x

[sebveit]

spring-security-crypto 7.1.0's AbstractValidatingPasswordEncoder#matches references org.springframework.util.StringUtils (in spring-core), which spring-security-crypto declares as an optional dependency. Gradle therefore does not resolve it transitively, so the class is absent from the test classpath and every SpringSecurityIntegrationTest case fails on a clean dependency resolution with NoClassDefFoundError (as seen in CI).

Add spring-core 7.0.8 (the Spring Framework 7.0.x line that Spring Security 7.1.0 targets) to the test classpath.

Description

Brief description of what this PR does.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issue

Fixes #65

Changes Made

• Add spring-core 7.0.8 (the Spring Framework 7.0.x line that Spring Security 7.1.0 targets) to the test classpath.

Testing

• Unit tests pass locally (./gradlew test)
• Integration tests pass (./gradlew test --tests "*IntegrationTest")
• New tests added for new functionality
• All tests achieve 100% pass rate

Test Coverage

• New code is covered by tests
• Edge cases are tested
• Error conditions are tested

Compatibility

• Verified compatibility with Spring Security
• Verified compatibility with Bouncy Castle
• No breaking changes to public API

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added Javadoc for all public methods
• I have updated the CHANGELOG.md if needed

Screenshots (if applicable)

Add screenshots here if UI/visual changes.

Additional Notes

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