PHPLIB-1715: Allow string to object conversion

[paulinevos]

Part of DRIVERS-3273

[Copilot]

Pull request overview

Adds builder support for the MongoDB aggregation $toObject expression operator (server feature noted as “New in MongoDB 8.3”), along with a fixture and test to validate pipeline rendering.

Changes:

• Introduces ToObjectOperator and exposes it via Expression::toObject(...).
• Adds a new pipeline JSON fixture for the “convert string to object” example.
• Adds a PHPUnit test asserting the rendered pipeline matches the fixture.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

File	Description
src/Builder/Expression/ToObjectOperator.php	New auto-generated operator class for $toObject.
src/Builder/Expression/FactoryTrait.php	Adds toObject() factory method to construct ToObjectOperator.
tests/Builder/Expression/Pipelines.php	Adds JSON fixture for $toObject conversion example.
tests/Builder/Expression/ToObjectOperatorTest.php	Adds coverage ensuring the builder emits the expected $toObject pipeline.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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