Clarify missing client binding errors

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 76f2163a-0773-4889-b627-1bd4e5c7bea0

Author: andystaples

azure-functions-durable/azure/durable_functions/decorators/durable_app.py

@@ -411,6 +411,10 @@ def attach_client_function(user_fn: Callable[..., Any]) -> FunctionBuilder:
             @wraps(function)
             async def client_bound(*args: Any, **kwargs: Any) -> Any:
                 bound = signature.bind(*args, **kwargs)
+                if client_name not in bound.arguments:
+                    raise TypeError(
+                        f"durable client binding parameter '{client_name}' is not "
+                        f"declared by function '{function.__name__}'")
                 raw_client = bound.arguments[client_name]
                 if not isinstance(raw_client, str):
                     raise TypeError(

tests/azure-functions-durable/test_decorator_compat.py

@@ -2,6 +2,7 @@
 # Licensed under the MIT License.
 
 import azure.durable_functions as df
+import pytest
 from azure.durable_functions.constants import (
     ACTIVITY_TRIGGER,
     DURABLE_CLIENT,
@@ -147,6 +148,17 @@ def starter(client):
     assert clients[0] is clients[1]
 
 
+async def test_durable_client_input_rejects_missing_binding_parameter():
+    app = df.DFApp()
+
+    async def starter(other):
+        return other
+
+    fb = app.durable_client_input(client_name="client")(starter)
+    with pytest.raises(TypeError, match="binding parameter 'client' is not declared"):
+        await fb._function._func(other="{}")
+
+
 # ---------------------------------------------------------------------------
 # All decorators register a function builder
 # ---------------------------------------------------------------------------