Respect global Python version pins in uv tool run and uv tool install (#14112)

Closes #12921

For `uv tool run`, we'll just use the global Python version for all
invocations without an explicit alternative request (i.e., via the
`--python` flag).

For `uv tool install`, it's a bit more complicated:

- If the tool is not installed, we'll use the global Python version
- If the tool is already installed, we won't change the Python version
unless `--reinstall` or `--python` is used
- If the tool was installed with `--python`, we won't use the global
Python version, unless the tool is uninstalled first

The behavior can be demonstrated as follows

```
$ uv python pin --global 3.12
$ uv tool install flask  # uses 3.12
$ uv tool install flask  # no-op
$ uv python pin --global 3.13
$ uv tool install flask  # no-op
$ uv tool install flask --reinstall  # uses 3.13
$ uv tool install flask -p 3.12 # uses 3.12
$ uv tool install flask  # no-op
$ uv tool install flask --reinstall # uses 3.12
```

This is a little more complicated than always reinstalling when the
global Python version pin changes, but I think it's probably more
intuitive when actually using the tool. We briefly touched on this when
adding global version pins at
https://github.com/astral-sh/uv/pull/12115#discussion_r1992222278

Minor note: I need to do a self-review of this implementation, as it's a
little awkward to encode this behavior in the existing logic.

crates/uv-python/src/discovery.rs

@@ -46,7 +46,7 @@ use crate::{BrokenSymlink, Interpreter, PythonVersion};
 /// A request to find a Python installation.
 ///
 /// See [`PythonRequest::from_str`].
-#[derive(Debug, Clone, PartialEq, Eq, Default, Hash)]
+#[derive(Debug, Clone, Eq, Default)]
 pub enum PythonRequest {
     /// An appropriate default Python installation
     ///
@@ -73,6 +73,18 @@ pub enum PythonRequest {
     Key(PythonDownloadRequest),
 }
 
+impl PartialEq for PythonRequest {
+    fn eq(&self, other: &Self) -> bool {
+        self.to_canonical_string() == other.to_canonical_string()
+    }
+}
+
+impl std::hash::Hash for PythonRequest {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.to_canonical_string().hash(state);
+    }
+}
+
 impl<'a> serde::Deserialize<'a> for PythonRequest {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where