Fix InvalidOperationException in Selectors when running parallel tests

[Copilot]

A common pattern we use in other places: https://github.com/search?q=repo%3Amicrosoft%2Fplaywright-dotnet+%2Fforeach.*s%5C.ToArray%5C%28%5C%29%5C%29%2F&type=code

Fixes a thread safety issue in Selectors.cs that causes System.InvalidOperationException: Collection was modified; enumeration operation may not execute when running tests in parallel.

Problem

The issue occurs in two methods in the Selectors class:

• SetTestIdAttribute() at line 70
• RegisterAsync() at line 56

Both methods enumerate over the _contextsForSelectors list while other threads may be modifying it (adding/removing browser contexts), causing the classic collection modification exception.

// This throws InvalidOperationException under parallel execution
foreach (var context in _contextsForSelectors)
{
    context.SendMessageToServerAsync("setTestIdAttributeName", ...);
}

The _contextsForSelectors list is modified by:

• Browser.ConnectToBrowserType() and Browser.DidCreateContext() - adds contexts
• BrowserContext.OnClose() - removes contexts

Solution

Create a snapshot of the list before enumeration using ToArray() to prevent the collection modification exception:

// Thread-safe enumeration using snapshot
foreach (var context in _contextsForSelectors.ToArray())
{
    context.SendMessageToServerAsync("setTestIdAttributeName", ...);
}

This approach:

• ✅ Fixes the race condition without breaking existing functionality
• ✅ Has minimal performance impact (only creates snapshot when contexts exist)
• ✅ Maintains thread safety for parallel test execution
• ✅ No changes to public API

Fixes #3207.

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