Cache MetadataReader by class name to avoid duplicate metadata

[LordKay-sudo]

Summary

Addresses duplicate metadata caching in CachingMetadataReaderFactory when the same .class file is discovered through different Resource representations during classpath scanning (e.g. ClassPathResource with a relative path vs UrlResource with a filesystem path).

This matches the duplication described by @pkernevez and hypothesized by @sbrannen in gh-36737, and corresponds to issue 1 as split by @bclozel in #36737 (comment).

• Adds a class-name keyed cache so only one MetadataReader (and its ClassFileAnnotationMetadata on Java 24+) is retained per class
• Restores class-name caching previously provided by Spring Boot's removed ConcurrentReferenceCachingMetadataReaderFactory (spring-boot#47687), while still delegating to the ClassFile-backed factory on Java 24+

What this does NOT fix

gh-36737 reports several independent memory problems. This PR does not address:

• ClassFileAnnotationMetadata using more memory per class than the ASM implementation (bclozel's issue 2; workaround: shadow MetadataReaderFactoryDelegate)
• Micrometer global registry pinning contexts (management.metrics.use-global-registry=false; Spring Boot)
• PostgreSQL JDBC SharedTimer pinning contexts via InheritableThreadLocals (reported by @pkernevez)
• Multiple cached ApplicationContext instances in large test suites

Reproduction

On main, the new tests fail:

CachingMetadataReaderFactoryTests > shouldNotDuplicateMetadataForSameClassFromDifferentResources() FAILED
    org.mockito.exceptions.verification.TooManyActualInvocations

CachingMetadataReaderFactoryTests > shouldReuseMetadataReaderForSameClassFromDifferentResources() FAILED
    java.lang.AssertionError

With this PR, all four tests in CachingMetadataReaderFactoryTests pass.

Test plan

• ./gradlew :spring-core:test --tests "org.springframework.core.type.classreading.CachingMetadataReaderFactoryTests"
• ./gradlew :spring-core:test --tests "org.springframework.core.type.classreading.*"
• Verified reproduction tests fail on main before applying the fix

Related to gh-36737