test: extract bash detection to shared _bash helper

Moves get_bash_executable and require_bash into a shared module so their
lru_cache state is per-session rather than per-test-module. Also adds a
comment regarding monkeypatching since lru_cache will silently drop changes
to os.environ if it has already been populated.

Closes follow-up items from PR review.

tests/_bash.py

@@ -0,0 +1,36 @@
+import functools
+import os
+import shutil
+import subprocess
+
+# Warning: get_bash_executable and require_bash use @functools.lru_cache.
+# Do not monkeypatch os.environ['PATH'] or shutil.which in tests and expect 
+# these functions to re-evaluate. If you must patch them, call .cache_clear()
+# before and after the test.
+
+@functools.lru_cache(maxsize=1)
+def get_bash_executable() -> str | None:
+    if os.name == 'nt':
+        for candidate in (
+            r'C:\Program Files\Git\bin\bash.exe',
+            r'C:\Program Files\Git\usr\bin\bash.exe',
+            r'C:\Program Files (x86)\Git\bin\bash.exe',
+            os.path.expandvars(r'%LOCALAPPDATA%\Programs\Git\bin\bash.exe'),
+        ):
+            if os.path.exists(candidate):
+                return candidate
+    bash = shutil.which('bash')
+    if bash and 'WindowsApps' not in bash:
+        return bash
+    return None
+
+@functools.lru_cache(maxsize=1)
+def require_bash() -> bool:
+    bash = get_bash_executable()
+    if not bash:
+        return False
+    try:
+        res = subprocess.run([bash, '-c', 'echo 1'], capture_output=True, text=True, timeout=2)
+        return res.returncode == 0
+    except Exception:
+        return False

tests/test_pre_push_hook_contract.py

@@ -5,34 +5,7 @@ import subprocess
 import unittest
 from pathlib import Path
 
-
-@functools.lru_cache(maxsize=1)
-def get_bash_executable() -> str | None:
-    if os.name == 'nt':
-        for candidate in (
-            r'C:\Program Files\Git\bin\bash.exe',
-            r'C:\Program Files\Git\usr\bin\bash.exe',
-            r'C:\Program Files (x86)\Git\bin\bash.exe',
-            os.path.expandvars(r'%LOCALAPPDATA%\Programs\Git\bin\bash.exe'),
-        ):
-            if os.path.exists(candidate):
-                return candidate
-    bash = shutil.which('bash')
-    if bash and 'WindowsApps' not in bash:
-        return bash
-    return None
-
-
-@functools.lru_cache(maxsize=1)
-def require_bash() -> bool:
-    bash = get_bash_executable()
-    if not bash:
-        return False
-    try:
-        res = subprocess.run([bash, '-c', 'echo 1'], capture_output=True, text=True, timeout=2)
-        return res.returncode == 0
-    except Exception:
-        return False
+from tests._bash import get_bash_executable, require_bash
 
 
 REPO_ROOT = Path(__file__).resolve().parents[1]

tests/test_roadmap_helpers.py

@@ -14,34 +14,7 @@ REPO_ROOT = Path(__file__).resolve().parents[1]
 NEXT_ID = REPO_ROOT / 'scripts' / 'roadmap-next-id.sh'
 DOGFOOD_PROBE = REPO_ROOT / 'scripts' / 'dogfood-probe.py'
 
-
-@functools.lru_cache(maxsize=1)
-def get_bash_executable() -> str | None:
-    if os.name == 'nt':
-        for candidate in (
-            r'C:\Program Files\Git\bin\bash.exe',
-            r'C:\Program Files\Git\usr\bin\bash.exe',
-            r'C:\Program Files (x86)\Git\bin\bash.exe',
-            os.path.expandvars(r'%LOCALAPPDATA%\Programs\Git\bin\bash.exe'),
-        ):
-            if os.path.exists(candidate):
-                return candidate
-    bash = shutil.which('bash')
-    if bash and 'WindowsApps' not in bash:
-        return bash
-    return None
-
-
-@functools.lru_cache(maxsize=1)
-def require_bash() -> bool:
-    bash = get_bash_executable()
-    if not bash:
-        return False
-    try:
-        res = subprocess.run([bash, '-c', 'echo 1'], capture_output=True, text=True, timeout=2)
-        return res.returncode == 0
-    except Exception:
-        return False
+from tests._bash import get_bash_executable, require_bash
 
 
 def run_next_id(roadmap: Path, script: Path = NEXT_ID) -> subprocess.CompletedProcess[str]: