Validate that discovered interpreters meet the Python preference (#7934)

Closes https://github.com/astral-sh/uv/issues/5144

e.g.

```
❯ cargo run -q -- sync --python-preference only-system
Using CPython 3.12.6 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Resolved 9 packages in 14ms
Installed 8 packages in 9ms
 + anyio==4.6.0
 + certifi==2024.8.30
 + h11==0.14.0
 + httpcore==1.0.5
 + httpx==0.27.2
 + idna==3.10
 + ruff==0.6.7
 + sniffio==1.3.1

❯ cargo run -q -- sync --python-preference only-managed
Using CPython 3.12.1
Removed virtual environment at: .venv
Creating virtual environment at: .venv
Resolved 9 packages in 14ms
Installed 8 packages in 11ms
 + anyio==4.6.0
 + certifi==2024.8.30
 + h11==0.14.0
 + httpcore==1.0.5
 + httpx==0.27.2
 + idna==3.10
 + ruff==0.6.7
 + sniffio==1.3.1
```

crates/uv-python/src/interpreter.rs

@@ -271,15 +271,28 @@ impl Interpreter {
     ///
     /// Returns `false` if we cannot determine the path of the uv managed Python interpreters.
     pub fn is_managed(&self) -> bool {
+        if let Ok(test_managed) =
+            std::env::var(uv_static::EnvVars::UV_INTERNAL__TEST_PYTHON_MANAGED)
+        {
+            // During testing, we collect interpreters into an artificial search path and need to
+            // be able to mock whether an interpreter is managed or not.
+            return test_managed.split_ascii_whitespace().any(|item| {
+                let version = <PythonVersion as std::str::FromStr>::from_str(item).expect(
+                    "`UV_INTERNAL__TEST_PYTHON_MANAGED` items should be valid Python versions",
+                );
+                if version.patch().is_some() {
+                    version.version() == self.python_version()
+                } else {
+                    (version.major(), version.minor()) == self.python_tuple()
+                }
+            });
+        }
+
         let Ok(installations) = ManagedPythonInstallations::from_settings(None) else {
             return false;
         };
 
-        installations
-            .find_all()
-            .into_iter()
-            .flatten()
-            .any(|install| install.path() == self.sys_base_prefix)
+        self.sys_base_prefix.starts_with(installations.root())
     }
 
     /// Returns `Some` if the environment is externally managed, optionally including an error