Remove VSCode extension install retry workaround, use insiders for all tests

[Copilot]

The retry loop workaround for microsoft/vscode#256031 is no longer needed as the issue has been fixed in vscode-insiders.

Changes

• test/vscodeLauncher.ts: Remove the 5-attempt retry loop with 2s delays in installExtensions()
• azure-pipelines.yml: Always use insiders instead of conditionally selecting stable/insiders based on build reason
• azure-pipelines/profiling.yml: Switch testVSCodeVersion from stable to insiders

Before

for (let attempts = 0; attempts < 5; attempts++) {
    try {
        await installExtension(extensionId, vscodeCli, vscodeArgs);
        installSucceeded = true;
        break;
    } catch (error) {
        console.warn(`Failed to install extension ${extensionId}; retrying: ${error}`);
        installError = error;
        await new Promise((resolve) => setTimeout(resolve, 2000));
    }
}

After

for (const extensionId of extensionIds) {
    await installExtension(extensionId, vscodeCli, vscodeArgs);
}

Original prompt

Remove the workaround here to call install extension in a loop - https://github.com/dotnet/vscode-csharp/blob/main/test/vscodeLauncher.ts#L72

The issue has been fixed in vscode-insiders. Update the azure-pipelines yaml and profiling yaml to use the insiders version

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