chore: enforce PLC0415 in tests for the remaining packages (#8547)

Follow-up to #8540, which turned on `PLC0415` (import-outside-top-level)
for checkpoint-postgres and checkpoint-sqlite. This does the remaining
six packages: checkpoint, checkpoint-conformance, langgraph, prebuilt,
cli, sdk-py.

Scoped to tests, per @sydney-runkle's call on #8540: library code is
exempted with `per-file-ignores`, since it still has deferred imports
nobody has reviewed and mixing that in would make this hard to read.

## What changed

Function-level imports across 56 test files moved to module level. Nine
could not move and carry an explicit `# noqa: PLC0415` with a reason:

| File | Why it stays local |
|---|---|
| `libs/langgraph/tests/test_deprecation.py` (4) | the import has to run
inside `pytest.warns` for the warning to be observed |
| `libs/langgraph/tests/test_serde_allowlist.py` | try/except guard,
skips when langchain_core is absent |
| `libs/langgraph/tests/test_delta_channel_benchmark.py` | optional
psycopg probe |
| `libs/checkpoint/tests/test_conformance_delta.py` (3) | protected by a
module-level `pytest.importorskip`; hoisting past the guard turns a skip
into a collection error |

That last one is the trap: an import moved above `pytest.importorskip`
silently defeats the guard. I hit it locally and it turned the skip into
a `ModuleNotFoundError` at collection. Every file with an `importorskip`
or `except ImportError` was checked by hand for this.

## Verification

`make lint` and `make test` in each of the six:

| Package | Tests |
|---|---|
| checkpoint | 156 passed, 17 skipped |
| checkpoint-conformance | 1 passed |
| langgraph | 1968 passed, 4 skipped |
| prebuilt | 284 passed |
| cli | 336 passed |
| sdk-py | 493 passed |

Also confirmed the rule actually fires: a throwaway test file with a
function-level import is flagged in all six packages, and the source
exemption holds.

libs/prebuilt/tests/test_tool_node.py

@@ -2,6 +2,7 @@ import contextlib
 import dataclasses
 import json
 import sys
+import warnings
 from functools import partial
 from typing import (
     Annotated,
@@ -23,10 +24,12 @@ from langchain_core.messages import (
 from langchain_core.runnables.config import RunnableConfig
 from langchain_core.tools import BaseTool, InjectedToolArg, ToolException
 from langchain_core.tools import tool as dec_tool
+from langchain_core.tools.base import InjectedToolCallId
 from langgraph.config import get_stream_writer
 from langgraph.errors import GraphBubbleUp, GraphInterrupt
 from langgraph.graph import START, MessagesState, StateGraph
 from langgraph.graph.message import REMOVE_ALL_MESSAGES, add_messages
+from langgraph.runtime import ExecutionInfo, ServerInfo
 from langgraph.store.base import BaseStore
 from langgraph.store.memory import InMemoryStore
 from langgraph.types import Command, Send
@@ -41,6 +44,7 @@ from langgraph.prebuilt import (
 )
 from langgraph.prebuilt.tool_node import (
     TOOL_CALL_ERROR_TEMPLATE,
+    ToolCallRequest,
     ToolInvocationError,
     ToolRuntime,
     tools_condition,
@@ -59,7 +63,6 @@ def _create_mock_runtime(store: BaseStore | None = None) -> Mock:
     which is injected by RunnableCallable from config["configurable"]["__pregel_runtime"].
     When testing ToolNode directly (outside a graph), we need to provide this manually.
     """
-    from langgraph.runtime import ExecutionInfo
 
     mock_runtime = Mock()
     mock_runtime.store = store
@@ -625,7 +628,6 @@ def test_tool_node_node_interrupt() -> None:
 
 @pytest.mark.parametrize("input_type", ["dict", "tool_calls"])
 async def test_tool_node_command(input_type: str) -> None:
-    from langchain_core.tools.base import InjectedToolCallId
 
     @dec_tool
     def transfer_to_bob(tool_call_id: Annotated[str, InjectedToolCallId]):
@@ -934,7 +936,6 @@ async def test_tool_node_command(input_type: str) -> None:
 
 
 async def test_tool_node_command_list_input() -> None:
-    from langchain_core.tools.base import InjectedToolCallId
 
     @dec_tool
     def transfer_to_bob(tool_call_id: Annotated[str, InjectedToolCallId]):
@@ -1194,7 +1195,6 @@ async def test_tool_node_command_list_input() -> None:
 
 
 def test_tool_node_parent_command_with_send() -> None:
-    from langchain_core.tools.base import InjectedToolCallId
 
     @dec_tool
     def transfer_to_alice(tool_call_id: Annotated[str, InjectedToolCallId]):
@@ -1282,7 +1282,6 @@ def test_tool_node_parent_command_with_send() -> None:
 
 
 async def test_tool_node_command_remove_all_messages() -> None:
-    from langchain_core.tools.base import InjectedToolCallId
 
     @dec_tool
     def remove_all_messages_tool(tool_call_id: Annotated[str, InjectedToolCallId]):
@@ -1621,9 +1620,6 @@ def test_tool_node_stream_writer() -> None:
 
 def test_tool_call_request_setattr_deprecation_warning():
     """Test that ToolCallRequest raises a deprecation warning on direct attribute modification."""
-    import warnings
-
-    from langgraph.prebuilt.tool_node import ToolCallRequest
 
     # Create a mock ToolCall
     tool_call = {"name": "test", "args": {"a": 1}, "id": "call_1", "type": "tool_call"}
@@ -2031,7 +2027,6 @@ def test_tool_runtime_defaults_tools_to_empty_list() -> None:
 
 def test_tool_runtime_forwards_execution_info_server_info_and_tools() -> None:
     """Test that execution_info, server_info, and tools are forwarded from Runtime to ToolRuntime."""
-    from langgraph.runtime import ExecutionInfo, ServerInfo
 
     exec_info = ExecutionInfo(
         thread_id="t-1",
@@ -2088,7 +2083,6 @@ async def test_tool_runtime_forwards_execution_info_server_info_and_tools_async(
     None
 ):
     """Test that execution_info, server_info, and tools are forwarded in async path."""
-    from langgraph.runtime import ExecutionInfo, ServerInfo
 
     exec_info = ExecutionInfo(
         thread_id="t-2",