Add MCP client conformance regression gates (#36810)

## What changed

- Add a harness that runs the Codex executable against a pinned official MCP
  client conformance suite across shipping, intermediate, and modern protocol
  versions, HTTP and stdio transports, and OAuth scenarios.
- Add a separate app-server regression matrix for transport, security, schema,
  pagination, SSE, multi-round request, and catalog-boundary behavior.
- Check both suites against committed baselines so previously passing or
  required checks cannot disappear or newly fail, while keeping known failures
  visible in complete reports.
- Run the fixture self-tests and both executable-level gates from the Python
  and TypeScript SDK test workflows.

## Testing

- Add unit tests for the fixture server, official-suite adapter, conformance
  runner, baseline comparison, and reviewer regression runner.

GitOrigin-RevId: de59f039294e34ed72873d9f6940b52e89172c0d

sdk/python/tests/test_mcp_conformance_fixtures.py

@@ -0,0 +1,37 @@
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+
+def test_mcp_conformance_fixture_self_tests() -> None:
+    repository_root = Path(__file__).resolve().parents[3]
+    candidates = (
+        repository_root / "public" / "scripts" / "mcp_conformance",
+        repository_root / "scripts" / "mcp_conformance",
+    )
+    fixture_directory = next((candidate for candidate in candidates if candidate.is_dir()), None)
+    assert fixture_directory is not None, (
+        "MCP conformance fixtures are missing; checked "
+        + " and ".join(str(candidate) for candidate in candidates)
+    )
+
+    env = os.environ.copy()
+    env["PYTEST_DISABLE_PLUGIN_AUTOLOAD"] = "1"
+    result = subprocess.run(
+        [sys.executable, "-m", "pytest", "-q", str(fixture_directory)],
+        cwd=repository_root,
+        env=env,
+        capture_output=True,
+        text=True,
+        timeout=120,
+        check=False,
+    )
+
+    assert result.returncode == 0, (
+        f"MCP conformance fixture self-tests failed with exit "
+        f"{result.returncode}.\n"
+        f"Fixture directory: {fixture_directory}\n"
+        f"STDOUT:\n{result.stdout}\n"
+        f"STDERR:\n{result.stderr}"
+    )