Fix codefix of analyzer for flowing cancellation token

[Copilot]

The FlowTestContextCancellationTokenFixer was generating invalid code when TestContext was not in scope, causing compilation errors (CS0103). The codefix would generate code like testContext.CancellationTokenSource.Token even when no testContext variable existed.

Changes Made

Custom FixAllProvider Implementation

• Added FlowTestContextCancellationTokenFixAllProvider using DocumentBasedFixAllProvider pattern
• Ensures TestContext property is added only once per class when multiple fixes are applied
• Coordinates multiple diagnostics in the same document efficiently

Enhanced Individual Codefix Logic

• Modified ApplyFixWithTestContextAsync to ensure TestContext availability before applying fixes
• For static methods (e.g., ClassCleanup, AssemblyCleanup): adds TestContext testContext parameter
• For instance methods: adds public TestContext TestContext { get; set; } property when missing
• Preserves existing TestContext references when already available

Before (broken):

[TestClass]
public class MyTestClass
{
    [ClassCleanup]
    public static async Task ClassCleanup()
    {
        await Task.Delay(1000, testContext.CancellationTokenSource.Token); // CS0103 error
    }
}

After (fixed):

[TestClass]
public class MyTestClass
{
    [ClassCleanup]
    public static async Task ClassCleanup(TestContext testContext)
    {
        await Task.Delay(1000, testContext.CancellationTokenSource.Token); // Valid code
    }
}

The fix ensures generated code always compiles successfully while making minimal necessary changes to add TestContext only when needed.

Fixes #6237.

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[Youssef1313]

/backport to rel/3.10

[github-actions]

Started backporting to rel/3.10: https://github.com/microsoft/testfx/actions/runs/16642926022