chore: enable RUF100 and clear unused noqa directives (#8546)

Follow-up to review on #8540, where a stale `# noqa: E402` slipped past
me and Sydney spotted it by eye. This turns on the rule that catches
that automatically.

`RUF100` flags a `noqa` that suppresses nothing. `sdk-py` already had it
through its blanket `RUF` selection; this adds it to the other seven
packages and clears what it finds.

### The 33 it flags, all autofixed

**Blanket `# noqa` on docstring-closing lines** (4, in
`checkpoint-postgres` and `checkpoint-sqlite`). `E501` is in
`lint.ignore` for those packages, so nothing was being suppressed:

```diff
-        """  # noqa
+        """
```

**`# noqa: F821` on `anext(aiter_)`** (2). Left over from Python 3.9
support. `anext` became a builtin in 3.10, which is the floor now, so
`F821` no longer fires:

```diff
-                    anext(aiter_),  # type: ignore[arg-type]  # noqa: F821
+                    anext(aiter_),  # type: ignore[arg-type]
```

**Suppressions naming rules the package does not enable** (27), across
`langgraph`, `prebuilt` and `checkpoint-sqlite`: `FBT001`, `FBT002`,
`TC002`, `BLE001`, `ANN001`, `ANN002`, `ANN003`, `E501`, `F401`. Mostly
copied between packages whose rule sets differ.

### One measurement note

If you check these numbers yourself, use `--extend-select`:

```
ruff check --select RUF100 .          # 81, misleading
ruff check --extend-select RUF100 .   # 33, real
```

With a bare `--select`, ruff treats every other rule as disabled, so
every suppression for another rule looks unused. I quoted 81 before
catching that.

### Verified

`checkpoint-sqlite` 118 passed, `prebuilt` 284 passed, `langgraph` 1968
passed, `checkpoint-postgres` 264 passed on PG 15 and 16. `make lint`
clean in every package.

Independent of #8540 and #8537, so it can land in any order.

libs/checkpoint-postgres/langgraph/checkpoint/postgres/__init__.py

@@ -223,7 +223,7 @@ class PostgresSaver(BasePostgresSaver):
             >>> checkpoint_tuple = memory.get_tuple(config)
             >>> print(checkpoint_tuple)
             CheckpointTuple(...)
-        """  # noqa
+        """
         thread_id = config["configurable"]["thread_id"]
         checkpoint_id = get_checkpoint_id(config)
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")

libs/checkpoint-postgres/langgraph/checkpoint/postgres/aio.py

@@ -573,7 +573,7 @@ class AsyncPostgresSaver(BasePostgresSaver):
         while True:
             try:
                 yield asyncio.run_coroutine_threadsafe(
-                    anext(aiter_),  # type: ignore[arg-type]  # noqa: F821
+                    anext(aiter_),  # type: ignore[arg-type]
                     self.loop,
                 ).result()
             except StopAsyncIteration:

libs/checkpoint-postgres/langgraph/checkpoint/postgres/shallow.py

@@ -334,7 +334,7 @@ class ShallowPostgresSaver(BasePostgresSaver):
             >>> checkpoint_tuple = memory.get_tuple(config)
             >>> print(checkpoint_tuple)
             CheckpointTuple(...)
-        """  # noqa
+        """
         thread_id = config["configurable"]["thread_id"]
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
         args = (thread_id, checkpoint_ns)
@@ -885,7 +885,7 @@ class AsyncShallowPostgresSaver(BasePostgresSaver):
         while True:
             try:
                 yield asyncio.run_coroutine_threadsafe(
-                    anext(aiter_),  # type: ignore[arg-type]  # noqa: F821
+                    anext(aiter_),  # type: ignore[arg-type]
                     self.loop,
                 ).result()
             except StopAsyncIteration: