test(reset): select a working Bash instead of an unusable WSL launcher (#671)

* test(reset): probe Bash variable expansion on Windows

Use a bounded variable-expansion probe so a launcher that starts successfully but consumes $x is rejected. Rebase over #676 and keep its upstream-checkout guard alongside Bash discovery.

Thanks to MadsLorentzen for diagnosing the launcher expansion failure and verifying the stronger probe.

* changelog: place the #671 entry under [Unreleased]

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Mads Lorentzen <madslorentzen_17@hotmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/_bash.py

@@ -0,0 +1,31 @@
+"""Find a usable Bash for tests that replay the documented reset commands."""
+import os
+from pathlib import Path
+import shutil
+import subprocess
+
+WINDOWS = os.name == "nt"
+
+
+def find_bash():
+    candidates = [shutil.which("bash")]
+    if WINDOWS:
+        # A Windows PATH may find the WSL launcher even without a usable WSL
+        # installation. Git for Windows also ships Bash beside its cmd/ entry
+        # points, but does not normally add that bin/ directory to PATH.
+        git = shutil.which("git")
+        if git:
+            candidates.append(str(Path(git).resolve().parent.parent / "bin" / "bash.exe"))
+    for candidate in dict.fromkeys(candidates):
+        if not candidate:
+            continue
+        try:
+            result = subprocess.run(
+                [candidate, "-c", 'x=ok; test "$x" = ok'],
+                stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, timeout=5,
+            )
+        except (OSError, subprocess.SubprocessError):
+            continue
+        if result.returncode == 0:
+            return candidate
+    return None