Writes retried on DEADLINE_EXCEEDED can double-apply

[zeevdr]

Description

sdk/src/opendecree/client.py:259-304 runs set / set_many / set_null through with_retry. SetField is not idempotent server-side (audit log, version bump). A retry on DEADLINE_EXCEEDED can double-apply if the server processed the write before the timeout.

Acceptance criteria

• Only retry reads (GetField, GetConfig, ListSchemas, ...) by default
• Writes require an explicit idempotency key argument to opt into retry
• Document retry semantics in API reference
• Add tests