GODRIVER-3890 Move aggregate to the mongo package.

[matthewdale]

GODRIVER-3890

Summary

Move the aggregate operation logic from the x/mongo/driver/operation package to the mongo package, removing the unnecessary chainable setters. Don't export the moved types.

Background & Motivation

Part of a larger effort to reduce unnecessary code by removing the x/mongo/driver/operation package.

[mongodb-drivers-pr-bot]

API Change Report

./v2/x/mongo/driver/operation

incompatible changes

Aggregate: removed
NewAggregate: removed

[mongodb-drivers-pr-bot]

🧪 Performance Results

Commit SHA: 46227b3

The following benchmark tests for version 6a55a900cfb66200078dfb56 had statistically significant changes (i.e., |z-score| > 1.96):

Benchmark	Measurement	% Change	Patch Value	Stable Region	H-Score	Z-Score
BenchmarkMultiInsertSmallDocument	ns_per_op	6.5896	6749.0000	Avg: 6331.7650 Med: 6325.0000 Stdev: 170.2519	0.7854	2.4507
BenchmarkMultiFindMany	ops_per_second_med	-2.7696	3649635.0365	Avg: 3753595.5644 Med: 3745318.3521 Stdev: 40933.7167	0.7908	-2.5397

For a comprehensive view of all microbenchmark results for this PR's commit, please check out the Evergreen perf task for this patch.

[matthewdale]

These tests are redundant with the TestCollection/aggregate integration tests.

[Copilot]

Pull request overview

Moves the low-level aggregate command construction/execution logic out of x/mongo/driver/operation into the public mongo package as an unexported operation type, and updates call sites to build the operation via struct literals rather than chainable setters.

Changes:

• Deleted x/mongo/driver/operation.Aggregate and introduced an unexported mongo.aggregateOp.
• Updated Collection aggregate/count and ChangeStream code paths to use aggregateOp directly.
• Removed the x/mongo/driver/integration aggregate integration test and an unused test helper.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
x/mongo/driver/operation/aggregate.go	Removed the old exported Aggregate operation implementation.
x/mongo/driver/integration/main_test.go	Removed autoInsertDocs helper that was only used by the deleted aggregate integration test.
x/mongo/driver/integration/aggregate_test.go	Removed integration tests that exercised operation.Aggregate directly.
mongo/op_aggregate.go	Added unexported aggregateOp implementation in the mongo package.
mongo/collection.go	Switched aggregate/count code paths to instantiate/configure aggregateOp via struct literals/field assignment.
mongo/change_stream.go	Switched change stream aggregate execution to use *aggregateOp instead of *operation.Aggregate.

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