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/pyproject.toml

@@ -32,6 +32,7 @@ test = [
   "pytest-mock",
   "psycopg[binary]",
   "langgraph-checkpoint",
+  "langgraph-checkpoint-conformance",
   "pytest-watcher",
 ]
 lint = [
@@ -49,6 +50,7 @@ default-groups = ['dev']
 
 [tool.uv.sources]
 langgraph-checkpoint = { path = "../checkpoint", editable = true }
+langgraph-checkpoint-conformance = { path = "../checkpoint-conformance", editable = true }
 
 [tool.hatch.build.targets.wheel]
 include = ["langgraph"]