docs(ipc): explain why _resolve_ctx imports SessionContext lazily

The `# noqa: PLC0415` flag silenced ruff's "import outside top-level"
lint but didn't say why the import was placed there in the first place.

Add a comment naming the actual reason: `datafusion/__init__.py`
imports `datafusion.ipc` before `datafusion.context`, so a module-top
import would force `datafusion.context` to load mid-init of
`datafusion.ipc`. The cycle is benign today (`context.py` only pulls
`expr.py` at module scope, and neither pulls `ipc.py` back), but a
single new import added to `context.py`'s transitive dependency tree
could turn this into a real circular import. Deferring keeps
`datafusion.ipc` import-order-independent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/datafusion/ipc.py

@@ -159,6 +159,13 @@ def _resolve_ctx(
     worker = get_worker_ctx()
     if worker is not None:
         return worker
+    # Lazy import: `datafusion/__init__.py` imports `datafusion.ipc`
+    # before `datafusion.context`, so a module-top import would force
+    # `datafusion.context` to load mid-init of `datafusion.ipc`. The
+    # cycle is benign today (context.py only pulls expr.py at module
+    # scope, neither pulls ipc.py back), but a single new import in
+    # context.py's transitive deps could turn it into a real cycle.
+    # Deferring keeps `datafusion.ipc` import-order-independent.
     from datafusion.context import SessionContext  # noqa: PLC0415
 
     return SessionContext.global_ctx()