Add predicate version of IOperation.Descendants and DescendantsAndSelf

[SkyDevLab]

Fixes #53621

Summary

Adds overloads for IOperation.Descendants and IOperation.DescendantsAndSelf accepting an optional Func<IOperation, bool>? descendIntoChildren predicate, as approved in API review (#53621).

API Changes

namespace Microsoft.CodeAnalysis.Operations
{
    public static class OperationExtensions
    {
        public static IEnumerable<IOperation> Descendants(this IOperation? operation, Func<IOperation, bool>? descendIntoChildren);
        public static IEnumerable<IOperation> DescendantsAndSelf(this IOperation? operation, Func<IOperation, bool>? descendIntoChildren);
    }
}

Details

• Updates internal OperationExtensions.Descendants traversal to check descendIntoChildren(operation) on the root (yielding the root if includeSelf: true, but pruning child traversal if false), and descendIntoChildren(current) before pushing children of traversed operations.
• Preserves existing behavior when descendIntoChildren is null (the parameterless overloads forward descendIntoChildren: null).
• Registers public symbols in src/Compilers/Core/Portable/PublicAPI.Unshipped.txt.
• Adds unit tests in OperationTests.cs verifying null operation handling, null predicate equivalence, lambda/local function pruning, and root pruning semantics.

Microsoft Reviewers: Open in CodeFlow

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[dotnet-policy-service]

This PR modifies public API files. Please follow the instructions at https://github.com/dotnet/roslyn/blob/main/docs/contributing/API%20Review%20Process.md for ensuring all public APIs are reviewed before merging.

[333fred]

Let's just directly assert the operations in the list.

[SkyDevLab]

Updated to directly assert the exact sequence of OperationKinds in prunedDescendants.