fix: recover partial streamed text in exception paths; drop redundant RuntimeError guard

- Strands agent: use accumulated full_text_parts in except block before
  falling back to the error string, so partial streamed output is not
  discarded when the generator raises mid-stream.
- CrewAI agent: same fix — prefer joined full_text_parts over the raw
  exception message when chunks were already emitted.
- memory.py.j2: remove the redundant `and not isinstance(exc, RuntimeError)`
  guard; RuntimeError is not an OSError subclass so the check was a no-op.

Agent-Logs-Url: https://github.com/neo4j-labs/create-context-graph/sessions/b196a73d-1eda-4b15-9309-f8a79e97ba70

Co-authored-by: johnymontana <1222454+johnymontana@users.noreply.github.com>