tsp - Use 'powershell' for @clientname decorator

[dolauli]

This pull request introduces targeted improvements to the PowerShell emitter and related utilities. The main changes focus on ensuring that the correct client name overrides are applied for PowerShell, and updating test logic to better handle file selection during compilation.

PowerShell-specific client name handling:

• Updated both getClientName in convertor.ts and getSchemaForModel in modelUtils.ts to pass "powershell" as the override context, ensuring client name overrides are correctly applied for PowerShell scenarios. [1] [2]

Test robustness improvements:

• Modified the GenerateModuleWithEmitter function in EmitterTest.ps1 to check for the existence of client.tsp and use it if available, otherwise defaulting to the current directory, making test compilation more flexible and robust.