Clarify 'uv python upgrade' message when no versions exist

[sinsniwal]

Summary

Resolves : #16783

That message is only visible if uv found no managed Python installations. If you had Python installed (even if it was fully up-to-date), requests would not be empty, and the code would skip this block and move on to the resolution phase.

The output message "There are no installed versions to upgrade" was ambiguous. It was unclear whether uv could not find any installed versions or if the existing versions were simply already up-to-date as mentioned in issue.

So updated the message to
"No Python installations found; run `uv python install` to install a Python version.
to explicitly indicate when no uv-managed Python installations exist.

Test Plan

Updated the existing test snapshot in python_upgrade_without_version to match the new output string. Verified that tests pass locally with cargo test.

[zanieb]

Is this case entirely redundant with the one above?

[sinsniwal]

Yes, makes sense. I've applied those changes in the new commit.

[zanieb]

I wonder if we should use our hint: ... styling for the second part of the message?

We should style the command with color like we do for other commands in output.

nit: I'd probably say "use" instead of "run".

[sinsniwal]

sure , i have added those changes in the new commit, thanks.

[zanieb]

The hint formatting I was referring to is like

uv/crates/uv-python/src/interpreter.rs

Lines 863 to 869 in 60a811e

	write!(
	f,
	"\n\n{}{} Consider recreating the environment (e.g., with `{}`)",
	"hint".bold().cyan(),
	":".bold(),
	"uv venv".green()
	)?;

[sinsniwal]

Made the changes