CSHARP-4901 Add support for SearchMeta facet without operator

[kyra-rk]

No description provided.

[Copilot]

Pull request overview

Adds the ability to build/render Atlas Search facet definitions without specifying an underlying search operator, enabling $searchMeta facet use cases that only bucket on facets.

Changes:

• Added SearchDefinitionBuilder.Facet(...) overloads that accept only facets (no operator).
• Updated facet operator rendering to omit the "operator" field when not provided.
• Added unit/integration tests covering facet-without-operator for both typed/untyped builders and $searchMeta.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/MongoDB.Driver/Search/SearchDefinitionBuilder.cs	Adds Facet(IEnumerable<SearchFacet<TDocument>>) and Facet(params SearchFacet<TDocument>[]) overloads to support operator-less facet definitions.
src/MongoDB.Driver/Search/OperatorSearchDefinitions.cs	Makes facet rendering conditional on whether an operator was provided (omit "operator" when null).
tests/MongoDB.Driver.Tests/Search/SearchDefinitionBuilderTests.cs	Adds rendering tests for facet definitions without an operator (typed and untyped).
tests/MongoDB.Driver.Tests/Search/AtlasSearchTests.cs	Adds an integration test verifying $searchMeta facet behavior without an operator.

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[Copilot]

Removing the non-null check for @operator changes behavior for the existing Facet(SearchDefinition<TDocument> @operator, ...) overloads: passing null previously threw ArgumentNullException, but now it silently renders a facet without an operator. To avoid masking bugs and preserve the prior contract, add a null check in the Facet overloads that take @operator (or restore validation in FacetSearchDefinition and introduce a separate code path/ctor for the operator-optional variant).

Suggested change

	_operator = @operator;
	_operator = Ensure.IsNotNull(@operator, nameof(@operator));

[kyra-rk]

Ah, this contradicts the original purpose of the ticket, which was to to make operator optional as per the Atlas Search docs.

[Copilot]

There’s an extra space after the opening parenthesis in subject.Facet( facetBuilder... which is inconsistent with the surrounding formatting in this test file and makes the call harder to scan. Consider removing the space for consistency.

Suggested change

	subject.Facet( facetBuilder.String("string", x => x.FirstName, 100)),
	expected);
	AssertRendered(
	subject.Facet( facetBuilder.String("string", "FirstName", 100)),
	subject.Facet(facetBuilder.String("string", x => x.FirstName, 100)),
	expected);
	AssertRendered(
	subject.Facet(facetBuilder.String("string", "FirstName", 100)),

[kyra-rk]

Will adjust the formatting to be on the next line!

[Copilot]

There’s an extra space after the opening parenthesis in subject.Facet( facetBuilder... which is inconsistent with the surrounding formatting in this test file and makes the call harder to scan. Consider removing the space for consistency.

Suggested change

	subject.Facet( facetBuilder.String("string", x => x.FirstName, 100)),
	expected);
	AssertRendered(
	subject.Facet( facetBuilder.String("string", "FirstName", 100)),
	subject.Facet(facetBuilder.String("string", x => x.FirstName, 100)),
	expected);
	AssertRendered(
	subject.Facet(facetBuilder.String("string", "FirstName", 100)),

[kyra-rk]

Will adjust the formatting to be on the next line!