feat: add db.collection.countDocuments() and estimatedDocumentCount() operation support

[h3n4l]

Summary

• Add support for db.collection.countDocuments() command
• Add support for db.collection.estimatedDocumentCount() command

countDocuments()

• Returns the count of documents matching a filter
• Supports optional filter argument with query operators
• Supports options: hint, limit, skip

estimatedDocumentCount()

• Returns a fast estimate of total document count using collection metadata
• Faster than countDocuments() for large collections as it doesn't scan documents

Syntax

// countDocuments
db.users.countDocuments()
db.users.countDocuments({ status: "active" })
db.users.countDocuments({ age: { $gte: 30 } })
db.users.countDocuments({}, { hint: "_id_" })
db.users.countDocuments({}, { hint: { status: 1 } })
db.users.countDocuments({}, { limit: 100, skip: 10 })

// estimatedDocumentCount
db.users.estimatedDocumentCount()

Test plan

• Integration tests for countDocuments with/without filter
• Integration tests for countDocuments with options (hint, limit, skip)
• Integration tests for estimatedDocumentCount
• All existing tests pass
• Linting clean

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds support for the db.collection.countDocuments() MongoDB operation, enabling users to count documents in a collection with optional filtering capabilities.

Changes:

• Added opCountDocuments operation type constant
• Implemented argument extraction for countDocuments with optional filter support
• Added executor function to handle countDocuments operations
• Comprehensive test coverage for various scenarios (no filter, with filter, empty collection, empty filter)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
translator.go	Added opCountDocuments operation type, implemented extractCountDocumentsArgs function to parse filter arguments, and integrated countDocuments into the method call visitor
executor.go	Added executeCountDocuments function to execute the count operation via MongoDB driver and integrated it into the operation dispatcher
executor_test.go	Added four comprehensive test cases covering basic usage, filtering with operators, empty collections, and empty filters

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