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/cli/pyproject.toml

@@ -72,10 +72,15 @@ lint.select = [
   "UP", # pyupgrade
   "B",  # flake8-bugbear
   "I",  # isort
+  "PLC0415",  # import-outside-top-level
   "RUF100",  # unused noqa directive
   "UP", # pyupgrade
 ]
 lint.ignore = ["E501", "B008"]
+# PLC0415 (import-outside-top-level) is enforced in tests only. Library code
+# still has deferred imports that have not been reviewed, so it stays exempt
+# for now.
+lint.per-file-ignores = { "langgraph_cli/**" = ["PLC0415"], "generate_schema.py" = ["PLC0415"] }
 target-version = "py310"
 
 [tool.ty.rules]