fix: prefer .cmd shim over POSIX wrapper for claude/codex on Windows

Fixes #695

`where claude` on Windows returns both the npm-generated POSIX shell
wrapper (extensionless) and the Windows batch shim (`claude.cmd`)
side-by-side. The existing `path_from_successful_output` helpers in
`claude_cli.rs` and `codex_cli.rs` picked the first existing line,
which gave the POSIX wrapper - Windows tried to load it as a PE32,
`CreateProcessW` returned ERROR_BAD_EXE_FORMAT (193), and the AI chat
panel surfaced "%1 is not a valid Win32 application."

Mirror the fix that landed for Gemini in `gemini_discovery.rs`: when
`cfg!(windows)`, filter candidates through the existing shared utility
`cli_agent_runtime::has_windows_cli_extension` so .bat/.cmd/.com/.exe
win over extensionless POSIX scripts. Non-Windows behavior is unchanged.

Added a regression test in each file matching the
`windows_path_lookup_prefers_cmd_shim_over_extensionless_npm_script`
pattern from gemini_discovery.rs.

src-tauri/src/codex_cli.rs

@@ -93,14 +93,25 @@ fn path_from_successful_output(output: &std::process::Output) -> Option<PathBuf>
 }
 
 fn first_existing_path(stdout: &str) -> Option<PathBuf> {
-    stdout.lines().find_map(|line| {
-        let trimmed = line.trim();
-        if trimmed.is_empty() {
-            return None;
-        }
-        let candidate = PathBuf::from(trimmed);
-        candidate.exists().then_some(candidate)
-    })
+    first_existing_path_for_platform(stdout, cfg!(windows))
+}
+
+fn first_existing_path_for_platform(stdout: &str, windows: bool) -> Option<PathBuf> {
+    let mut paths = stdout.lines().filter_map(existing_path);
+    if windows {
+        return paths.find(|path| crate::cli_agent_runtime::has_windows_cli_extension(path));
+    }
+
+    paths.next()
+}
+
+fn existing_path(line: &str) -> Option<PathBuf> {
+    let trimmed = line.trim();
+    if trimmed.is_empty() {
+        return None;
+    }
+    let candidate = PathBuf::from(trimmed);
+    candidate.exists().then_some(candidate)
 }
 
 fn codex_binary_candidates() -> Vec<PathBuf> {
@@ -1003,6 +1014,22 @@ printf '%s\n' '{"type":"item.completed","item":{"id":"msg_1","type":"agent_messa
         assert_eq!(first_existing_path(&stdout), Some(codex));
     }
 
+    #[test]
+    fn windows_path_lookup_prefers_cmd_shim_over_extensionless_npm_script() {
+        let dir = tempfile::tempdir().unwrap();
+        let shell_script = dir.path().join("codex");
+        let cmd_shim = dir.path().join("codex.cmd");
+        std::fs::write(&shell_script, "#!/bin/sh\n").unwrap();
+        std::fs::write(&cmd_shim, "@ECHO off\n").unwrap();
+
+        let stdout = format!("{}\n{}\n", shell_script.display(), cmd_shim.display());
+
+        assert_eq!(
+            first_existing_path_for_platform(&stdout, true),
+            Some(cmd_shim)
+        );
+    }
+
     #[cfg(unix)]
     #[test]
     fn command_path_from_shell_finds_codex_from_login_shell() {