tests: cover m2pack cli and mcp additions

Add discovery-helper tests (env var override, PATH fallback, missing
binary) and command tests that point M2PACK_BINARY at a Python stub
script echoing canned JSON per subcommand. Cover success paths,
non-zero exit, non-JSON output, JSON-array output, and missing binary.
Extend the MCP schema mirror test to cover all 12 tools and add
dispatch tests for the new m2pack_* argv translation.

tests/test_m2pack_binary.py

@@ -0,0 +1,68 @@
+"""Tests for the m2pack binary discovery helper."""
+
+from __future__ import annotations
+
+import os
+import stat
+from pathlib import Path
+
+import pytest
+
+from metin_release.errors import ValidationError
+from metin_release.m2pack_binary import ENV_VAR, resolve_m2pack_binary
+
+
+def _make_stub(tmp_path: Path, name: str = "m2pack") -> Path:
+    stub = tmp_path / name
+    stub.write_text("#!/bin/sh\necho '{}'\n")
+    stub.chmod(stub.stat().st_mode | stat.S_IEXEC | stat.S_IXGRP | stat.S_IXOTH)
+    return stub
+
+
+def test_env_var_override_wins(tmp_path: Path, monkeypatch):
+    stub = _make_stub(tmp_path)
+    # Scrub PATH so shutil.which can't resolve m2pack
+    monkeypatch.setenv("PATH", "/nonexistent")
+    monkeypatch.setenv(ENV_VAR, str(stub))
+    resolved = resolve_m2pack_binary()
+    assert resolved == stub.resolve()
+
+
+def test_env_var_empty_falls_through_to_path(tmp_path: Path, monkeypatch):
+    stub = _make_stub(tmp_path)
+    monkeypatch.setenv(ENV_VAR, "   ")  # blank-ish
+    monkeypatch.setenv("PATH", str(tmp_path))
+    resolved = resolve_m2pack_binary()
+    assert resolved == stub.resolve()
+
+
+def test_env_var_pointing_nowhere_raises(tmp_path: Path, monkeypatch):
+    monkeypatch.setenv(ENV_VAR, str(tmp_path / "does-not-exist"))
+    monkeypatch.setenv("PATH", "/nonexistent")
+    with pytest.raises(ValidationError) as exc:
+        resolve_m2pack_binary()
+    assert exc.value.error_code == "m2pack_not_found"
+
+
+def test_path_fallback_used_when_env_unset(tmp_path: Path, monkeypatch):
+    stub = _make_stub(tmp_path)
+    monkeypatch.delenv(ENV_VAR, raising=False)
+    monkeypatch.setenv("PATH", str(tmp_path))
+    resolved = resolve_m2pack_binary()
+    assert resolved == stub.resolve()
+
+
+def test_missing_binary_raises_validation_error(monkeypatch):
+    monkeypatch.delenv(ENV_VAR, raising=False)
+    monkeypatch.setenv("PATH", "/nonexistent")
+    with pytest.raises(ValidationError) as exc:
+        resolve_m2pack_binary()
+    assert exc.value.error_code == "m2pack_not_found"
+    assert "M2PACK_BINARY" in str(exc.value)
+
+
+def test_custom_env_mapping_parameter(tmp_path: Path):
+    stub = _make_stub(tmp_path)
+    fake_env = {ENV_VAR: str(stub)}
+    resolved = resolve_m2pack_binary(env=fake_env)
+    assert resolved == stub.resolve()