fix(cursor): avoid waiting on buffering to run aggregation middleware

[vkarpov15]

Summary

I was working on #16284, spotted a potential cause of flaking tests: aggregation cursor waits on buffering to run middleware, which is inconsistent with other places where we run middleware regardless of buffering. That is the likely cause for this test flake.

I also cranked up a timeout to see whether this failure was due to a buffering timeout or just slowness on the MongoDB server.

Examples

[Copilot]

Pull request overview

This PR targets CI flakiness around aggregation cursors by changing when aggregation middleware runs relative to collection buffering, and by increasing a test suite timeout to help diagnose slow/buffering-related failures.

Changes:

• Run pre('aggregate') middleware immediately when creating an AggregationCursor, instead of waiting for the collection buffer to drain.
• Increase the Mocha timeout for aggregate.exec() tests to 15s.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
lib/cursor/aggregationCursor.js	Adjusts aggregation cursor initialization to execute pre aggregate middleware without waiting on collection buffering.
test/aggregate.test.js	Increases timeout for aggregate exec tests to reduce/diagnose CI flakes due to slowness/buffering.

[AbdelrahmanHafez]

We have a few flaky runs in the last commit:
https://github.com/Automattic/mongoose/actions/runs/26375366547/job/77634594787?pr=16289
https://github.com/Automattic/mongoose/actions/runs/26375366545/job/77634594737?pr=16289
https://github.com/Automattic/mongoose/actions/runs/26375367241/job/77634596554?pr=16289

Probably worth investigating. Otherwise the code changes LGTM