fix(compass-aggregations): exclude database-level operators from AI prompts and autocomplete COMPASS-10677

[ivandevp]

Description

When users ask the AI assistant to generate an aggregation pipeline with prompts like "Create a pipeline to generate 20 documents describing different roasted coffee beans", the AI would suggest $documents (and other database-level stages). These fail at runtime because they require {aggregate: 1} (database-level aggregation), not collection-level:

'$documents' can only be run with {aggregate: 1}

The text editor's autocomplete also surfaced these operators, compounding the confusion.

Two fixes:

1. AI prompt — added an explicit instruction to buildInstructionsForAggregateQuery() prohibiting database-level stages ($documents, $changeStream, $currentOp, $listLocalSessions, etc.) and explaining the pipeline runs against a collection.

2. Autocomplete — wired the existing stage.namespace filter (already used by the stage wizard) through to createAggregationAutocompleter, so the text editor no longer suggests operators whose namespaces is ['db'].

Checklist

• New tests and/or benchmarks are included
• Documentation is changed or added
• If this change updates the UI, screenshots/videos are added and a design review is requested
• If this change could impact the load on the MongoDB cluster, please describe the expected and worst case impact
• I have signed the MongoDB Contributor License Agreement (https://www.mongodb.com/legal/contributor-agreement)

Motivation and Context

• Bugfix
• New feature
• Dependency update
• Misc

Fixes COMPASS-10677.

Open Questions

N/A

Dependents

N/A

Types of changes

• Backport Needed
• Patch (non-breaking change which fixes an issue)
• Minor (non-breaking change which adds functionality)
• Major (fix or feature that would cause existing functionality to change)