Agent mode: Re-write cd && commands when they are not necessary

[Tyriar]

This is a very common occurrence currently:

It makes sense that the LLM would do this to make sure it's in the right cwd, but since we often know the cwd if we could re-write it before the run in terminal UI is shown to the user it would be a nice thing to do.

[Tyriar]

I don't think this is possible without having the chatSessionId in LanguageModelToolInvocationPrepareOptions since we don't know the terminal that will be used without that.

[Tyriar]

Need to fix a unit test that incorrectly passes -Path from https://github.com/microsoft/vscode-copilot-chat/pull/46/files

[Tyriar]

To verify please test this on Window with pwsh and mac/linux. Some examples to try:

• pwsh only: Set-Location -Path <cwd>; foo
• pwsh only: Set-Location -Path "<cwd>" && foo
• pwsh only: cd <cwd>; foo
• cd <cwd> && foo