feat(checkpoint-postgres): add extended methods and conformance tests

Implements adelete_for_runs, acopy_thread, aprune (and sync counterparts)
for AsyncPostgresSaver and PostgresSaver. Adds conformance test harness
using langgraph-checkpoint-conformance as an editable dev dependency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

libs/checkpoint-postgres/tests/test_conformance.py

@@ -0,0 +1,56 @@
+"""Conformance tests for AsyncPostgresSaver."""
+# mypy: disable-error-code="import-untyped"
+
+from __future__ import annotations
+
+from collections.abc import AsyncGenerator
+from uuid import uuid4
+
+import pytest
+from langgraph.checkpoint.conformance import checkpointer_test, validate
+from langgraph.checkpoint.conformance.report import ProgressCallbacks
+from psycopg import AsyncConnection
+from psycopg.rows import dict_row
+
+from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
+from tests.conftest import DEFAULT_POSTGRES_URI
+
+
+async def pg_lifespan() -> AsyncGenerator[None, None]:
+    """No-op lifespan; databases are created per-checkpointer instance."""
+    yield
+
+
+@checkpointer_test(name="AsyncPostgresSaver", lifespan=pg_lifespan)
+async def postgres_checkpointer() -> AsyncGenerator[AsyncPostgresSaver, None]:
+    database = f"test_{uuid4().hex[:16]}"
+    async with await AsyncConnection.connect(
+        DEFAULT_POSTGRES_URI, autocommit=True
+    ) as conn:
+        await conn.execute(f"CREATE DATABASE {database}")
+    try:
+        async with await AsyncConnection.connect(
+            DEFAULT_POSTGRES_URI + database,
+            autocommit=True,
+            prepare_threshold=0,
+            row_factory=dict_row,
+        ) as conn:
+            saver = AsyncPostgresSaver(conn)
+            await saver.setup()
+            yield saver
+    finally:
+        async with await AsyncConnection.connect(
+            DEFAULT_POSTGRES_URI, autocommit=True
+        ) as conn:
+            await conn.execute(f"DROP DATABASE {database}")
+
+
+@pytest.mark.asyncio
+async def test_full_conformance() -> None:
+    """AsyncPostgresSaver passes ALL conformance tests."""
+    report = await validate(
+        postgres_checkpointer,
+        progress=ProgressCallbacks.verbose(),
+    )
+    report.print_report()
+    assert report.passed_all(), f"Conformance failed: {report.to_dict()}"