test: use runtime bash check instead of platform check

Replace sys.platform == 'win32' with an actual bash invocation test
to handle environments where bash exists but is non-functional (e.g.,
WSL stub on Windows without an installed distro).

Author: mnriem

tests/conftest.py

@@ -1,14 +1,30 @@
 """Shared test helpers for the Spec Kit test suite."""
 
 import re
-import sys
+import shutil
+import subprocess
 
 import pytest
 
 _ANSI_ESCAPE_RE = re.compile(r"\x1b\[[0-?]*[ -/]*[@-~]")
 
+
+def _has_working_bash() -> bool:
+    """Check whether a functional bash is available (not just a WSL stub)."""
+    if shutil.which("bash") is None:
+        return False
+    try:
+        r = subprocess.run(
+            ["bash", "-c", "echo ok"],
+            capture_output=True, text=True, timeout=5,
+        )
+        return r.returncode == 0 and "ok" in r.stdout
+    except (OSError, subprocess.TimeoutExpired):
+        return False
+
+
 requires_bash = pytest.mark.skipif(
-    sys.platform == "win32", reason="bash not available on Windows"
+    not _has_working_bash(), reason="working bash not available"
 )