Add @SpringConversion negative test

[scordio]

This adds a negative test for when spring-core is not in the classpath.

[Copilot]

Pull request overview

Adds a regression/negative integration test to ensure @SpringConversion fails in a predictable way when spring-core is absent from the runtime classpath, using a custom classloader to simulate that environment.

Changes:

• Added a negative integration test case that runs a parameterized test under a classloader that blocks org.springframework.core.*.
• Extended the local JUnit Platform testkit helper to support selecting tests using a provided ClassLoader.

Reviewed changes

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

File	Description
src/test/java/io/github/scordio/tests/junit/converters/SpringConversionIntegrationTests.java	Adds a new negative test and a custom URLClassLoader that filters out Spring Core classes.
src/test/java/io/github/scordio/tests/junit/converters/JupiterEngineTestKit.java	Adds an overload to execute a test class via JUnit Platform using a specific ClassLoader.

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