Fix CancellationToken code fixer to handle named arguments properly

[Copilot]

The FlowTestContextCancellationTokenFixer was not correctly handling methods with named arguments when adding CancellationToken parameters. This resulted in invalid C# syntax being generated.

Problem

When applying the code fixer to methods with optional parameters, the fixer would generate invalid code:

public void DoSomething(string x = "", int i = 1, CancellationToken ct = default) {}

[TestMethod]
public void A()
{
    DoSomething(); // Fixed as DoSomething(TestContext.CancellationToken); ❌ Invalid - skips optional params
    DoSomething(i: 15); // Fixed as DoSomething(i: 15, TestContext.CancellationToken); ❌ Invalid - mixes named/positional
}

Solution

Enhanced the AddCancellationTokenArgument method to:

1. Analyze method signatures using the semantic model to find CancellationToken parameters and their names
2. Determine appropriate argument style based on existing arguments and parameter positions
3. Use named arguments when necessary to maintain valid C# syntax

The fixer now generates correct code:

DoSomething(); // → DoSomething(ct: TestContext.CancellationToken); ✅
DoSomething(i: 15); // → DoSomething(i: 15, ct: TestContext.CancellationToken); ✅
DoSomething("test"); // → DoSomething("test", ct: TestContext.CancellationToken); ✅
DoSomething("test", 42); // → DoSomething("test", 42, TestContext.CancellationToken); ✅

Key Changes

• Smart argument detection: Uses ShouldUseNamedArgument helper to determine when named arguments are required
• Parameter name preservation: Correctly uses the actual parameter name (e.g., ct, cancellationToken)
• Backward compatibility: Maintains existing behavior for Task.Delay and other common scenarios
• Comprehensive test coverage: Added test cases covering various argument combinations

The fix ensures the code fixer generates syntactically correct and consistent C# code that follows language conventions.

Fixes #6500.

Warning

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[Youssef1313]

Suggested change

	editor.ReplaceNode(invocationExpression, (node, generator) =>
	editor.ReplaceNode(invocationExpression, (node, _) =>