feat(client-python): add write_array_elements default-fill helper and document semantics

Regenerate Python proto bindings to pick up MxSparseArray/MxSparseElement/
sparse_array_value from the shared mxaccess_gateway.proto. Add
Session.write_array_elements which builds an MxValue(sparse_array_value=…)
from a {index→scalar} dict and delegates to the existing write(). Add 8 pytest
tests covering builder correctness and full round-trip wire shape. Update
README with a default-fill semantics paragraph and bare-name array-write note.

clients/python/src/zb_mom_ww_mxgateway/session.py

@@ -489,6 +489,56 @@ class Session:
             correlation_id=correlation_id,
         )
 
+    async def write_array_elements(
+        self,
+        server_handle: int,
+        item_handle: int,
+        element_data_type: "pb.MxDataType.ValueType",
+        total_length: int,
+        elements: dict[int, MxValueInput],
+        *,
+        user_id: int = 0,
+        correlation_id: str = "",
+    ) -> None:
+        """Write a partial array by specifying only the indices you want to set.
+
+        The gateway expands the sparse representation into a full ``total_length``
+        array before forwarding the write to MXAccess.  Indices not listed in
+        *elements* are filled with the type default for *element_data_type* (0,
+        False, empty string, Unix epoch for timestamps, etc.).  The previous
+        value at those positions is **not** preserved — this is a full array
+        replacement, not a patch.
+
+        Args:
+            server_handle: Handle returned by :meth:`register`.
+            item_handle: Handle returned by :meth:`add_item`.
+            element_data_type: ``pb.MX_DATA_TYPE_*`` enum value for the scalar
+                element type of the target array attribute.
+            total_length: Total number of elements in the written array.  Must
+                be > 0 and large enough to contain every index in *elements*.
+            elements: Mapping of zero-based element index to scalar value.
+                Values are converted with :func:`~zb_mom_ww_mxgateway.values.to_mx_value`.
+            user_id: Galaxy user id to stamp on the write (requires a prior
+                supervisory advise to take effect — see README).
+            correlation_id: Optional client-supplied correlation token echoed
+                in the command reply.
+        """
+        sparse = pb.MxSparseArray(
+            element_data_type=element_data_type,
+            total_length=total_length,
+            elements=[
+                pb.MxSparseElement(index=idx, value=to_mx_value(val))
+                for idx, val in elements.items()
+            ],
+        )
+        await self.write(
+            server_handle,
+            item_handle,
+            pb.MxValue(sparse_array_value=sparse),
+            user_id=user_id,
+            correlation_id=correlation_id,
+        )
+
     async def write2(
         self,
         server_handle: int,