Replace LINQ Any+Single patterns with Where+FirstOrDefault for better performance

[Copilot]

This PR addresses a performance optimization opportunity identified in PR #5717 where the pattern of using Any() followed by Single() with the same predicate can be improved.

Problem

The existing code uses this pattern in multiple places:

if (collection.Any(x => x.Uid == item.Uid))
{
    var existing = collection.Single(x => x.Uid == item.Uid);
    // handle duplicate...
}

This approach has two issues:

1. Performance: It enumerates the collection twice with the same predicate
2. Debuggability: When Single() throws, it's harder to debug since you can't inspect the actual duplicate items

Solution

Replace the pattern with:

var existing = collection.Where(x => x.Uid == item.Uid).FirstOrDefault();
if (existing is not null)
{
    // handle duplicate...
}

Benefits

• Performance: Single enumeration instead of two separate LINQ operations
• Debuggability: Better error context - developers can inspect the actual duplicate object when debugging
• Maintainability: More explicit null checking following modern C# best practices

Changes

Updated 14 instances across 3 files:

• TestHostOrchestratorManager.cs (2 instances)
• TestHostControllersManager.cs (6 instances)
• TestHostManager.cs (6 instances)

All changes maintain identical functional behavior while improving performance and debugging experience.

Fixes #5739.

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[Evangelink]

Let's capture all extensions with the same UID. Let's update PlatformResources.ExtensionWithSameUidAlreadyRegisteredErrorMessage message so that it accepts the list of types with duplicated UID.

[Evangelink]

Let's introduce a validation helper to factorize all these common checks

[Copilot]

Let's introduce a validation helper to factorize all these common checks

Implemented in commit 1a2b3c4. Created ExtensionValidationHelper to factorize all duplicate extension validation logic across the 14 instances in TestHostManager, TestHostControllersManager, and TestHostOrchestratorManager.