Resolve Client.Python-001/002/004/006/007/008/010/011/012 findings

Client.Python-001: dropped "scaffold" from the stale pyproject description.
Client.Python-002 (re-triaged): stale finding — MxGatewayCommandError is
already exported and in __all__; no change needed.
Client.Python-004: removed the dead `closed` variable in _smoke; the CLI
smoke now uses `async with session`.
Client.Python-006: close() on both clients and Session had an unlocked
check-then-set race; `_closed` is now set before the await.
Client.Python-007: gateway stream iterators now share one helper that
explicitly catches CancelledError and cancels the call.
Client.Python-008: to_mx_value now rejects nan/inf; float/bytes mapping
documented.
Client.Python-010: removed the circular-import-workaround late imports in
favour of TYPE_CHECKING / module-scope imports.
Client.Python-011: ensure_mxaccess_success no longer treats a proto3-default
success==0 with an unset category as a failure.
Client.Python-012 (Won't Fix): invoke_raw deliberately skips MXAccess-failure
detection for parity tests; documented the contract instead.

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

clients/python/src/mxgateway_cli/commands.py

@@ -18,6 +18,7 @@ from mxgateway.client import GatewayClient
 from mxgateway.errors import MxGatewayError
 from mxgateway.generated import mxaccess_gateway_pb2 as pb
 from mxgateway.options import ClientOptions
+from mxgateway.session import Session
 from mxgateway.values import MxValueInput
 
 MAX_AGGREGATE_EVENTS = 10_000
@@ -383,8 +384,7 @@ async def _write2(**kwargs: Any) -> dict[str, Any]:
 async def _smoke(**kwargs: Any) -> dict[str, Any]:
     async with await _connect(kwargs) as client:
         session = await client.open_session(client_session_name=kwargs["client_name"])
-        closed = False
-        try:
+        async with session:
             server_handle = await session.register(kwargs["client_name"])
             item_handle = await session.add_item(server_handle, kwargs["item"])
             await session.advise(server_handle, item_handle)
@@ -399,9 +399,6 @@ async def _smoke(**kwargs: Any) -> dict[str, Any]:
                 "itemHandle": item_handle,
                 "events": [_message_dict(event) for event in events],
             }
-        finally:
-            if not closed:
-                await session.close()
 
 
 async def _connect(kwargs: dict[str, Any]) -> GatewayClient:
@@ -419,9 +416,7 @@ async def _connect(kwargs: dict[str, Any]) -> GatewayClient:
     )
 
 
-def _session(client: GatewayClient, session_id: str):
-    from mxgateway.session import Session
-
+def _session(client: GatewayClient, session_id: str) -> Session:
     return Session(client=client, session_id=session_id)