Handle non-PTY case for modal shell and modal container exec

[aksh-at]

Resolves CLI-239

modal container exec now checks stdout.isatty() to determine whether to run in PTY mode or not. If not, it uses StreamType.STDOUT to write output directly to stdout. This means that you can now do this:

modal shell -c 'uv pip list' > env.txt

It didn't make sense for ContainerProcess.attach() to support pty=False, so I removed that option and made it a deprecation_error directly.

Added a test for modal container exec since none existed, but it's a bit tricky to test PTY behavior directly because of how we run this. Tested some basic cases manually:

Depends on #2624.

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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Changelog

• modal container exec and modal shell now work correctly even when a pseudoterminal (PTY) is not present. This means, for example, that you can pipe the output of these commands to a file:

  modal shell -c 'uv pip list' > env.txt

[pawalt]

Nice!

[pawalt]

Yeah maybe it should go in its own stream. The API is implemented like this to match subprocess, but we could easily add StreamType.STDERR.