Fix BucketOperationSupport _id field exposure.

[this-amine]

Expose _id field in BucketOperation and BucketAutoOperation to match MongoDB behavior.

MongoDB's $bucket operation always generates _id field with bucket boundaries,
but BucketOperation.getFields() only exposed output fields like 'count'.
This caused IllegalArgumentException when subsequent operations referenced _id.

• Modified BucketOperationSupport.asExposedFields() to always expose _id
• Removed defensive fallback in ProjectionOperation that masked the issue
• Updated and added comprehensive tests for both scenarios
• Added integration test proving bucket + addFields now works

Closes #5046

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).