CSHARP-6133: Validate cursor argument in IAsyncCursor/IAsyncCursorSource Extensions terminal operators

[adelinowona]

No description provided.

[Copilot]

Pull request overview

This PR tightens argument validation for IAsyncCursor and IAsyncCursorSource extension “terminal operators” (and related ForEachAsync overloads), ensuring ArgumentNullException is thrown with consistent parameter names and that invalid inputs don’t trigger unnecessary work.

Changes:

• Added Ensure.IsNotNull(...) checks to IAsyncCursorSourceExtensions terminal operators and ForEachAsync overloads so null source/processor is rejected early.
• Added Ensure.IsNotNull(...) checks to IAsyncCursorExtensions terminal operators, and adjusted ForEachAsync to preserve async-dispose behavior even when processor is null.
• Added/expanded unit tests covering null argument behavior for cursor/cursor-source terminal operators and ForEachAsync overload variants.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/MongoDB.Driver.Tests/Core/IAsyncCursorSourceExtensionsTests.cs	Adds coverage asserting null source/processor behavior and ensuring queries aren’t executed when processor is null.
tests/MongoDB.Driver.Tests/Core/IAsyncCursorExtensionsTests.cs	Adds coverage asserting null cursor behavior and verifies disposal behavior when processor is null.
src/MongoDB.Driver/Core/IAsyncCursorSource.cs	Introduces explicit null checks for cursor-source terminal operators and ForEachAsync overloads.
src/MongoDB.Driver/Core/IAsyncCursor.cs	Introduces explicit null checks for cursor terminal operators and refines ForEachAsync null-processor handling to preserve disposal semantics.

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