chore(checkpoint-postgres,checkpoint-sqlite): enable PLC0415 lint rule (#8540)

Follow-up to review on #8537: turn on ruff's `PLC0415`
(`import-outside-top-level`) so deferred imports in tests stop
accumulating.

Scoped to `checkpoint-postgres` and `checkpoint-sqlite` rather than
repo-wide, because the sweep turns up three different things and only
one of them is a style problem.

### What the rule finds today

```
package                 tests   src    files
checkpoint                13     10      11
checkpoint-conformance     0     10       4
checkpoint-postgres        6      0       2
checkpoint-sqlite          9      0       3
langgraph                130     23      32
prebuilt                  14      3       7
cli                        9     14      10
sdk-py                   189     23      38
                        ────────────────────
                         370     83     107
```

453 violations across 107 files, and ruff has no autofix for this rule.

### Three categories, not one

**Style — hoist.** `checkpoint-sqlite/tests/test_store.py` deferred
`math`, `random`, `time`, `Counter` and `defaultdict` inside methods for
no reason.

**Deliberate — keep, annotate.**
`checkpoint-postgres/tests/test_async.py` defers behind
`pytest.importorskip("langgraph.channels.delta")` because langgraph core
is *not* a test dependency of that package. Hoisting would break the
skip. Those get `# noqa: PLC0415` and a comment.

**Redundant guard — hoist.**
`checkpoint-sqlite/tests/test_conformance_delta.py` deferred imports
only to get past its own `importorskip`. Imports move up; the
`aiosqlite` guard stays, since that dependency genuinely can be absent.

The second category is why I did not enable this everywhere in one go.
Most of the 83 source-level violations look like the same pattern —
optional-dependency handling and circular-import avoidance in
`jsonplus.py`, `embed.py`, `encrypted.py` and friends. Blanket-enabling
would mean `# noqa` on a lot of correct code, and each one wants an
owner's eye rather than a mechanical pass.

These two packages are clean to enforce today because both have **zero**
source-level violations.

### Suggested rollout for the rest

Either extend package by package as owners confirm which deferrals are
intentional, or enable everywhere at once with `per-file-ignores`
grandfathering the current 107 files so new code is blocked immediately
and the debt burns down. Happy to do either — the second is a smaller
diff but leaves a long ignore list.

### Verified

`checkpoint-sqlite` 118 passed, `checkpoint-postgres` 264 passed on PG
15 and 16, `make lint` clean in both.

One overlap worth flagging:
`checkpoint-sqlite/tests/test_conformance_delta.py` is also touched by
#8537. The change is identical in both, so it should merge cleanly
either way.

libs/checkpoint-postgres/pyproject.toml

@@ -64,6 +64,7 @@ lint.select = [
   "UP", # pyupgrade
   "B",  # flake8-bugbear
   "I",  # isort
+  "PLC0415",  # import-outside-top-level
   "UP", # pyupgrade
 ]
 lint.ignore = ["E501", "B008"]

libs/checkpoint-postgres/tests/test_async.py

@@ -380,13 +380,15 @@ async def test_delta_channel_chain_reconstruction(saver_name: str) -> None:
         "langgraph.channels.delta", reason="langgraph core not installed"
     )
 
-    from typing import Annotated
+    # Deferred on purpose: langgraph core is not a test dependency of this
+    # package, so these must stay behind the importorskip above.
+    from typing import Annotated  # noqa: PLC0415
 
-    from langchain_core.messages import AIMessage, HumanMessage
-    from langgraph.channels.delta import DeltaChannel
-    from langgraph.graph import START, StateGraph
-    from langgraph.graph.message import _messages_delta_reducer
-    from typing_extensions import TypedDict
+    from langchain_core.messages import AIMessage, HumanMessage  # noqa: PLC0415
+    from langgraph.channels.delta import DeltaChannel  # noqa: PLC0415
+    from langgraph.graph import START, StateGraph  # noqa: PLC0415
+    from langgraph.graph.message import _messages_delta_reducer  # noqa: PLC0415
+    from typing_extensions import TypedDict  # noqa: PLC0415
 
     class State(TypedDict):
         messages: Annotated[list, DeltaChannel(_messages_delta_reducer)]

libs/checkpoint-sqlite/pyproject.toml

@@ -62,6 +62,7 @@ lint.select = [
   "UP", # pyupgrade
   "B",  # flake8-bugbear
   "I",  # isort
+  "PLC0415",  # import-outside-top-level
   "UP", # pyupgrade
 ]
 lint.ignore = ["E501", "B008"]

libs/checkpoint-sqlite/tests/test_conformance_delta.py

@@ -10,14 +10,14 @@ pytest.importorskip(
 )
 pytest.importorskip("aiosqlite", reason="aiosqlite not installed")
 
+from langgraph.checkpoint.conformance import validate  # noqa: E402
+from langgraph.checkpoint.conformance.initializer import checkpointer_test  # noqa: E402
+
+from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver  # noqa: E402
+
 
 @pytest.mark.asyncio
 async def test_delta_channel_conformance():
-    from langgraph.checkpoint.conformance import validate
-    from langgraph.checkpoint.conformance.initializer import checkpointer_test
-
-    from langgraph.checkpoint.sqlite.aio import AsyncSqliteSaver
-
     @checkpointer_test(name="AsyncSqliteSaver")
     async def sqlite_saver():
         async with AsyncSqliteSaver.from_conn_string(":memory:") as saver:

libs/checkpoint-sqlite/tests/test_store.py

@@ -1,7 +1,11 @@
+import math
 import os
+import random
 import re
 import tempfile
+import time
 import uuid
+from collections import Counter, defaultdict
 from collections.abc import Generator, Iterable
 from contextlib import contextmanager
 from typing import Any, Literal, cast
@@ -33,10 +37,6 @@ class CharacterEmbeddings(Embeddings):
 
     def __init__(self, dims: int = 50, seed: int = 42):
         """Initialize with embedding dimensions and random seed."""
-        import math
-        import random
-        from collections import defaultdict
-
         self._rng = random.Random(seed)
         self.dims = dims
         # Create projection vector for each character lazily
@@ -48,9 +48,6 @@ class CharacterEmbeddings(Embeddings):
 
     def _embed_one(self, text: str) -> list[float]:
         """Embed a single text."""
-        import math
-        from collections import Counter
-
         counts = Counter(text)
         total = sum(counts.values())
 
@@ -338,8 +335,6 @@ class TestSqliteStore:
 
             # Test update
             # Small delay to ensure the updated timestamp is different
-            import time
-
             time.sleep(0.01)
 
             updated_value = {"title": "Updated Document", "content": "Hello, Updated!"}