chore: raise clearer error when aget_messages is missing in memory

[italojohnny]

Add an explicit AttributeError when aget_messages is not found in the external memory object, helping users diagnose the issue.

[codspeed-hq]

CodSpeed Performance Report

Merging #6795 will degrade performances by 35.94%

_{Comparing chore/missing-aget_messages-check (c2a93fa) with main (8f276ad)}

Summary

❌ 1 regressions
✅ 13 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_build_flow_invalid_job_id	7.9 ms	12.3 ms	-35.94%