Add helpful error message for 'uv activate'

[nooscraft]

When users try to run 'uv activate', show a helpful error message
instead of the generic unknown command error. The message explains
that 'uv activate' is not supported and provides instructions on
how to activate a virtual environment.

This is most probably address the issue: #16993

[nooscraft]

@charliermarsh @zanieb FYI!

[nooscraft]

   error `uv activate` is not supported. To activate a virtual environment, use:
     source .venv/bin/activate

   hint Create a virtual environment with: uv venv

[darth-raijin]

Small suggestion about some possibly redundant comments

[darth-raijin]

Nit: The code already makes this intent pretty clear, so these two comments might be redundant. Fine to keep, but you could consider removing.

[zanieb]

I'm still not sure we should do this, but if we do, we probably need to actually use our shell detection logic as well as discover the virtual environment to ensure one exists?

[darth-raijin]

+1 on the above.

It feels like it could set a precedent for adding similar custom messages for other unsupported commands, instead of handling them in a more general way.

[nooscraft]

Hey, updated this based on @zanieb's feedback!

Now it actually detects your shell and finds the venv instead of just printing static text. Walks up from cwd looking for .venv/pyvenv.cfg, checks ZSH_VERSION/FISH_VERSION/etc to figure out what shell you're in, and gives you the right command.

Quick examples:

$ uv activate
error `uv activate` is not supported. To activate a virtual environment, use:
  source .venv/bin/activate

If you're in fish:

$ uv activate
error `uv activate` is not supported. To activate a virtual environment, use:
  source .venv/bin/activate.fish

No venv? Shows a hint:

$ uv activate
error `uv activate` is not supported. To activate a virtual environment, use:
  source .venv/bin/activate

hint Create a virtual environment with: uv venv

@darth-raijin - fair point on precedent, but we already do this for install, show, freeze etc (they suggest uv pip <cmd>). This one's a bit different since there's no actual command to suggest - just need to explain why uv activate can't be a thing and point them to the shell command.