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

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[christophstrobl]

Thank you for your contribution @this-amine. Merged to main development line and back ported to 4.5.x.