fix(messages): return errors on load failure

[MicroMilo]

Summary

• return HTTP 500 when the session messages read path fails
• include the error while preserving empty message metadata for response-shape compatibility

Verification

• npm --workspace ui test
• npm exec -- tsc --noEmit -p tsconfig.json

Notes

This is a real API semantics bug: the previous route converted read failures into HTTP 200 empty message lists, making failures indistinguishable from an empty session.