fix(tools): find Python on Windows without python3.exe

Installing debugpy on Windows failed with "cannot run python3.exe … (os error 3)": the tool
manager looked for `python3` first, and on Windows that only finds the App Execution Alias in
WindowsApps, which can't start without the Store's Python. Windows installs `python.exe` and the
`py` launcher instead. On Windows the interpreter is now looked up as `python`, `py`, then
`python3`, and a WindowsApps alias only counts when it runs. The debugpy adapter's default
interpreter is `python` on Windows too. macOS and Linux keep `python3` first.

crates/ide-dap/src/adapter.rs

@@ -72,7 +72,13 @@ fn builtin(adapter: &str) -> Option<DebuggerConfig> {
             "stdio",
             "Install the Kotlin Debug Adapter from Settings > Language Servers",
         ),
-        "debugpy" => cfg("python3", &["-m", "debugpy.adapter"], "stdio", "pip install debugpy"),
+        // Windows installs `python.exe`; `python3.exe` there is at most a Store alias.
+        "debugpy" => cfg(
+            if cfg!(windows) { "python" } else { "python3" },
+            &["-m", "debugpy.adapter"],
+            "stdio",
+            "pip install debugpy",
+        ),
         "js-debug" => cfg(
             "js-debug-adapter",
             // Without a host it listens on `localhost`, which may resolve to ::1 only.

crates/ide-tools/src/lib.rs

@@ -395,6 +395,11 @@ pub fn parse_version(text: &str) -> Option<String> {
     None
 }
 
+/// A Windows App Execution Alias (`%LOCALAPPDATA%\Microsoft\WindowsApps\python.exe`).
+fn is_app_execution_alias(path: &Path) -> bool {
+    path.parent().and_then(Path::file_name).is_some_and(|d| d.eq_ignore_ascii_case("WindowsApps"))
+}
+
 /// A tool whose "command" is a plugin jar (e.g. a jdtls bundle) rather than an executable.
 fn is_bundle(command: &str) -> bool {
     command.ends_with(".jar")
@@ -491,8 +496,21 @@ impl Tools {
         cmd
     }
 
+    /// The interpreter that installs and probes Python tools. Windows installs `python.exe`
+    /// and the `py` launcher, not `python3.exe`; its `WindowsApps` entries are App Execution
+    /// Aliases that only open the Store unless the Store's Python is installed, so they count
+    /// only when they run.
     fn python(&self) -> Option<PathBuf> {
-        self.which_any(&["python3", "python"])
+        if self.platform.os != Os::Windows {
+            return self.which_any(&["python3", "python"]);
+        }
+        ["python", "py", "python3"].into_iter().filter_map(|c| self.which(c)).find(|p| {
+            !is_app_execution_alias(p) || {
+                let mut c = self.python_command(p);
+                c.arg("--version");
+                run_quiet(c, Duration::from_secs(5)).is_some_and(|(ok, _)| ok)
+            }
+        })
     }
 
     /// Executable `cmd` inside `dir`, honouring Windows suffixes.

crates/ide-tools/src/tests.rs

@@ -700,6 +700,28 @@ fn rust_analyzer_managed_copy_survives_rustup_becoming_available() {
     assert!(!env.root.join("rust-analyzer").exists());
 }
 
+/// Windows has `python.exe` and `py`, not `python3.exe`; the `WindowsApps` aliases only count
+/// when the Store's Python behind them runs.
+#[cfg(unix)]
+#[test]
+fn windows_python_skips_store_aliases_that_do_not_run() {
+    let env = Env::new();
+    let aliases = env.sys.parent().unwrap().join("Microsoft").join("WindowsApps");
+    env.script(&aliases, "python", "exit 9009");
+    env.script(&aliases, "python3", "exit 9009");
+    env.script(&env.sys, "py", "echo Python 3.12.0");
+    let windows = Platform { os: Os::Windows, arch: platform::Arch::X64 };
+    let found = || {
+        let dirs = std::env::join_paths([aliases.clone(), env.sys.clone()]).unwrap();
+        let python = Tools::with_paths(env.root.clone(), dirs, windows).python().unwrap();
+        let dir = python.parent().unwrap().file_name().unwrap().to_string_lossy().into_owned();
+        format!("{dir}/{}", python.file_name().unwrap().to_string_lossy())
+    };
+    assert_eq!(found(), "sysbin/py");
+    env.script(&aliases, "python", "echo Python 3.13.0");
+    assert_eq!(found(), "WindowsApps/python", "the Store's Python runs");
+}
+
 #[cfg(unix)]
 #[test]
 fn python_probes_run_in_an_empty_private_directory() {