ask the Pants PEX for its version in test framework (#15)

Query the Pants PEX for a specific major/minor version for its version
instead of having the test hard-code the Pants versions. This means that
the test will just discover whatever Pants version is in the relevant
resolve automatically and is one less item to update when updating Pants
versions.

Author: tdyas

src/python/shoalsoft/pants_mcp_plugin/plugin_integration_test.py

@@ -99,13 +99,15 @@ def run_pants_without_waiting(
 
 
 @contextmanager
-def isolated_pants(pants_version_str: str):
-    pants_version = Version(pants_version_str)
-    pants_major_minor = f"{pants_version.major}.{pants_version.minor}"
+def isolated_pants(pants_major_minor: str):
+    pants_major_minor_version = Version(pants_major_minor)
+    assert (
+        len(pants_major_minor_version.release) == 2
+    ), "Expected only major/minor version to be provided."
 
     # Find the Python interpreter compatible with this version of Pants.
     py_version_for_pants_major_minor = (
-        "3.11" if Version(pants_major_minor) >= Version("2.25") else "3.9"
+        "3.11" if pants_major_minor_version >= Version("2.25") else "3.9"
     )
     python_path = python_interpreter_path(py_version_for_pants_major_minor)
     assert (
@@ -141,6 +143,17 @@ def isolated_pants(pants_version_str: str):
 
     buildroot = (Path.cwd() / f"buildroot-{pants_major_minor}").resolve()
     buildroot.mkdir(parents=True)
+    (buildroot / "BUILDROOT").touch()
+
+    # Determine the full version of the Pants used for the test.
+    version_result = subprocess.run(
+        [python_path, str(pants_pex_path), "--version"],
+        env={"NO_SCIE_WARNING": "1"},
+        capture_output=True,
+        check=True,
+        cwd=buildroot,
+    )
+    pants_version = Version(version_result.stdout.decode("utf-8").strip())
 
     workdir_base = buildroot / ".pants.d" / "workdirs"
     workdir_base.mkdir(parents=True)
@@ -262,7 +275,7 @@ async def get_pants_target_resource(spec: str) -> dict[str, typing.Any]:
     assert test_tgt["address"] == "//:test_tgt"
 
 
-@pytest.mark.parametrize("pants_version_str", ["2.29.0a0"])
+@pytest.mark.parametrize("pants_version_str", ("2.29",))
 def test_mcp_server_tools(pants_version_str: str) -> None:
     with isolated_pants(pants_version_str) as context:
         sources = {