Chat placeholder not responsive to smaller height windows

[mrleemurray]

Content should respond to smaller height viewports.

Screen Recording:

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[bpasero]

I think this is what @eli-w-king works on in #269791

[bpasero]

To be fair, I would like us to move away from special rendering of Chat OOTB, which is in https://github.com/microsoft/vscode-internalbacklog/issues/6000