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/README.md

@@ -148,6 +148,33 @@ the unchanged elements included. For example, to change 2 elements of a
 the 2 new ones). Sending only the 2 changed values overwrites the attribute
 with a 2-element array.
 
+### Default-fill partial array writes
+
+`Session.write_array_elements` lets you write only the indices you care about.
+The gateway fills every unmentioned position with the type default for the
+declared `element_data_type` (0, `False`, `""`, Unix epoch for timestamps).
+The previous value at those positions is **not** preserved — the gateway expands
+the sparse map to a full array before forwarding the write to MXAccess, so this
+is still a full replacement:
+
+```python
+# Write indices 0 and 5 of a 10-element integer array.
+# Positions 1-4 and 6-9 become 0, not their previous values.
+await session.write_array_elements(
+    server_handle=server_handle,
+    item_handle=item_handle,
+    element_data_type=pb.MX_DATA_TYPE_INTEGER,
+    total_length=10,
+    elements={0: 100, 5: 500},
+)
+```
+
+Bare-name array items (e.g. `Object.ArrayAttr` without an index suffix) added
+via `add_item` auto-normalize to `[]` — they refer to the whole array, not a
+single element. Writes through such handles must cover the full array or use
+`write_array_elements` to supply `total_length` and let the gateway fill
+defaults for the rest.
+
 ## Galaxy Repository Browse
 
 The `GalaxyRepositoryClient` wraps the read-only `GalaxyRepository` gRPC