artifact: in-memory service shares Part pointers between save, storage and load #1785

The in-memory artifact service stores the saved *genai.Part by pointer and returns that same pointer from Load (artifact/inmemory.go). Changing a part after saving it, or changing a part returned by Load, silently rewrites the stored artifact for every later reader.

adk-python deep-copies the part on save and on load (artifacts/in_memory_artifact_service.py).

Fix with regression tests: #1782

Written with the help of an agent (Claude Code, claude-opus-5-5).