Handle resizing in pty-shell

[azliu0]

Describe your changes

Very minor QoL improvement: support resizing the terminal window in pty-shell.

Implementation detail

It'd be cleaner to create the WindowSizeHandler inside of the ContainerProcess handle, but signal modifications must happen in the main thread / event loop. Synchronicity forces all apis wrapped in synchronize_api to execute in a different thread, hence the need to create WindowSizeHandler externally and pass it in as an arg to attach.

Server pr

• Pairs with server changes https://github.com/modal-labs/modal/pull/19216

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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