Support unordered Collection properties.

[lsh1215]

Summary

This change lets Spring Data JDBC represent unordered aggregate child collections that still allow duplicate elements by treating Collection differently from ordered List properties.

Root cause

BasicRelationalPersistentProperty previously classified every collection-like property except Set as list-like. As a result, Collection<Child> was treated as qualified and ordered, requiring an index/key column and using list-style generated-id aggregation.

Changes

• Restrict list-like detection to arrays and List implementations.
• Add a JDBC-side CollectionAggregator so generated child ids can be populated for unordered Collection properties without a qualifier.
• Add mapping metadata coverage for Collection being collection-like but not qualified or ordered.
• Add a JDBC integration regression test that saves and reloads duplicate child elements through Collection<CollectionElement> without a key column.
• Add DB-specific schema fixtures for the new integration-test tables.

Verification

• git diff --check upstream/main..HEAD
• ./mvnw -pl spring-data-relational,spring-data-jdbc -Dtest=DefaultAggregatePathUnitTests,JdbcRepositoryWithListsIntegrationTests,JdbcRepositoryWithCollectionsIntegrationTests test

Closes #848.

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[schauder]

Thanks.

I moved the test so Single Query Loading gets tested as well.
And I added a little documentation.

This is merged.