fix(terminal): also set MSYS2_ARG_CONV_EXCL for MSYS2/Cygwin bash fallback

MSYS_NO_PATHCONV is honored by Git for Windows bash only. _find_bash's
final shutil.which fallback can return MSYS2-proper or Cygwin bash,
which ignore it and honor MSYS2_ARG_CONV_EXCL instead. Set both so argv
path conversion stays disabled regardless of which bash flavor spawns.
Also subsumes the cmd /c mangling in #56147.

tests/tools/test_local_env_windows_msys.py

@@ -260,6 +260,23 @@ class TestWindowsMsysPathconvDefaults:
         run_env = _make_run_env({"MSYS_NO_PATHCONV": "0"})
         assert run_env.get("MSYS_NO_PATHCONV") == "0"
 
+    def test_msys2_arg_conv_excl_set_on_windows(self, monkeypatch):
+        # MSYS2-proper / Cygwin bash ignore MSYS_NO_PATHCONV; they honor
+        # MSYS2_ARG_CONV_EXCL. Both must be set on every env builder.
+        monkeypatch.setattr(local_mod, "_IS_WINDOWS", True)
+        assert _make_run_env({}).get("MSYS2_ARG_CONV_EXCL") == "*"
+        assert _sanitize_subprocess_env({}).get("MSYS2_ARG_CONV_EXCL") == "*"
+        assert hermes_subprocess_env().get("MSYS2_ARG_CONV_EXCL") == "*"
+
+    def test_msys2_arg_conv_excl_not_set_on_posix(self, monkeypatch):
+        monkeypatch.setattr(local_mod, "_IS_WINDOWS", False)
+        assert "MSYS2_ARG_CONV_EXCL" not in _make_run_env({})
+
+    def test_msys2_arg_conv_excl_respects_user_override(self, monkeypatch):
+        monkeypatch.setattr(local_mod, "_IS_WINDOWS", True)
+        run_env = _make_run_env({"MSYS2_ARG_CONV_EXCL": "/custom"})
+        assert run_env.get("MSYS2_ARG_CONV_EXCL") == "/custom"
+
 
 # ---------------------------------------------------------------------------
 # Command wrapping — native Windows cwd must be Git Bash-friendly for cd

tools/environments/local.py

@@ -759,10 +759,18 @@ def _apply_windows_msys_bash_env_defaults(env: dict) -> None:
     runs terminal commands through bash on Windows, so set the standard MSYS
     opt-out by default.  Users who need conversion can override in their env.
     Refs #56700.
+
+    ``MSYS_NO_PATHCONV`` is honored by Git for Windows bash only.  MSYS2-proper
+    and Cygwin bash (which ``_find_bash`` can still return via the final
+    ``shutil.which`` fallback) ignore it and honor ``MSYS2_ARG_CONV_EXCL``
+    instead, so set both.  ``*`` disables all argv conversion — the semantic
+    equivalent of ``MSYS_NO_PATHCONV=1``.  Also fixes ``cmd /c`` mangling
+    (#56147).
     """
     if not _IS_WINDOWS:
         return
     env.setdefault("MSYS_NO_PATHCONV", "1")
+    env.setdefault("MSYS2_ARG_CONV_EXCL", "*")
 
 
 def _path_env_key(run_env: dict) -> str | None: