azure-functions-durable: native sync durable client binding (SyncDurableFunctionsClient) #181

Summary

Add a first-class synchronous durable-client binding to azure-functions-durable so history export and scheduled-tasks code can obtain a sync durabletask client directly, instead of building one from the async binding.

Background

durable_client_input injects the async DurableFunctionsClient (an AsyncTaskHubGrpcClient). But several features use the sync durabletask clients:

• history-export activities use a sync TaskHubGrpcClient;
• ScheduledTaskClient and ExportHistoryClient are built on the sync client.

Today, route handlers and the export activities build a sync client from the async binding's config (rpcBaseUrl / taskHubName / requiredQueryStringParameters) via an async→sync adapter (_sync_client / _build_sync_client). That adapter is duplicated and a bit awkward.

Proposal

Add a sync client binding that injects a sync client built from the same host client-config JSON, removing the adapter. A sync consumer still needs the Functions conveniences (create_http_management_payload, create_check_status_response), so the injected type should be a real SyncDurableFunctionsClient carrying those helpers — not a bare TaskHubGrpcClient.

Naming: keep DurableFunctionsClient = async (avoids a breaking rename / v1 compat), add SyncDurableFunctionsClient. Retire the sync client once durabletask ships async ScheduledTaskClient / ExportHistoryClient.

Groundwork already merged (PR #155)

• _add_rich_client is generic over the client constructor.
• wrap_activity leaves native multi-binding activities untouched (so an activity can carry a durable-client binding).
• The durable-client middleware invokes synchronous user functions (awaits only awaitable results).

So the remaining work is: a sync client class + a sync binding decorator + lifecycle/caching design.

Open design questions

• Decorator shape: durable_client_input(..., sync=True) vs a separate durable_client_input_sync (the injected type differs).
• Which Functions helpers / v1 shims the sync client carries.
• Per-process caching and gRPC channel lifecycle (export fans out many activity invocations).

Context

Follow-up from #155.