fix(client/grpc): use SyncGrpcClient._create_channel in sync paths

[alvinttang]

Summary

SyncGrpcClient.from_url and SyncGrpcClient.wait_until_server_ready resolved _create_channel via the wrapper class GrpcClient, which only composes a sync + async grpc client and does not define _create_channel itself. Every sync-kind grpc client construction therefore raised:

AttributeError: type object 'GrpcClient' has no attribute '_create_channel'

before any RPC was attempted. Even if the lookup had fallen back to the async version, that returns grpc.aio.insecure_channel(...) which only supports async with — incompatible with the surrounding sync with block.

Fixes #4683

Root cause

Only the leaf classes SyncGrpcClient and AsyncGrpcClient define their own static _create_channel. GrpcClient (the wrapper) does not. The two sync paths referenced GrpcClient._create_channel directly.

Fix

src/bentoml/_internal/client/grpc.py:

• SyncGrpcClient.from_url is a @classmethod, so it now uses cls._create_channel(...) — that resolves to SyncGrpcClient._create_channel for the base class and lets subclasses override the channel factory.
• SyncGrpcClient.wait_until_server_ready is a @staticmethod (no cls), so it now hardcodes SyncGrpcClient._create_channel(...) — same observable behaviour as before, just calling a method that actually exists.

Tests

tests/unit/_internal/client/test_grpc_sync_create_channel.py:

• test_grpc_client_has_no_create_channel — guards against future regressions that re-introduce the missing attribute on the wrapper class.
• test_sync_grpc_client_create_channel_returns_sync_context_manager — the resolved helper must return a sync context manager.
• test_wait_until_server_ready_does_not_raise_create_channel_attribute_error — wait_until_server_ready must not raise the original AttributeError when the helper is not present on the wrapper.
• test_from_url_does_not_raise_create_channel_attribute_error — same for from_url.

Locally on darwin/arm64:

• pytest tests/unit/_internal/client/test_grpc_sync_create_channel.py → 4 passed.
• ruff check clean.

Risk notes

Two single-line call-site swaps inside SyncGrpcClient. Async paths already used AsyncGrpcClient._create_channel and are untouched. No public API change.