Introduce explicit index field matching methods

[DragonFSKY]

Closes #5187

Keeps the existing subset-matching behavior for compatibility, deprecates the ambiguous method, and adds explicit methods for:

• subset matching regardless of position
• exact field-name matching regardless of order
• compound-index prefix coverage

The internal index assertion now uses the explicit subset method.

Validation:

• ./mvnw -pl spring-data-mongodb -Dtest=IndexInfoUnitTests clean test (14 tests passed)

Checklist:

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided by the project and have not submitted unrelated formatting changes.
• You submit test cases that back your changes.
• You added yourself as author in the headers of the classes you touched.