Ensure `python pin --global` creates parent directories if missing (#12180)

## Summary

Closes https://github.com/astral-sh/uv/issues/12178

## Test Plan

Added new test. Manually tested on Windows and Linux.

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv/src/commands/python/pin.rs

@@ -144,6 +144,7 @@ pub(crate) async fn pin(
         let Some(config_dir) = user_uv_config_dir() else {
             return Err(anyhow::anyhow!("No user-level config directory found."));
         };
+        fs_err::tokio::create_dir_all(&config_dir).await?;
         PythonVersionFile::new(config_dir.join(PYTHON_VERSION_FILENAME))
             .with_versions(vec![request])
     } else {

crates/uv/tests/it/python_pin.rs

@@ -328,6 +328,31 @@ fn python_pin_global_use_local_if_available() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn python_pin_global_creates_parent_dirs() {
+    let context: TestContext = TestContext::new_with_versions(&["3.12"]);
+    let uv_global_config_dir = context.user_config_dir.child("uv");
+
+    assert!(
+        !uv_global_config_dir.exists(),
+        "Global config directory should not exist yet."
+    );
+
+    uv_snapshot!(context.filters(), context.python_pin().arg("3.12").arg("--global"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Pinned `[UV_USER_CONFIG_DIR]/.python-version` to `3.12`
+
+    ----- stderr -----
+    ");
+
+    assert!(
+        uv_global_config_dir.exists(),
+        "Global config directory should be automatically created (if missing) after global pin."
+    );
+}
+
 /// We do not need a Python interpreter to pin without `--resolved`
 /// (skip on Windows because the snapshot is different and the behavior is not platform dependent)
 #[cfg(unix)]