fix: show alternate streamed error text safely

[AetherAI3]

Summary

Some streamed failure frames carry a user-facing error string instead of msg or message. The 0.4 client discarded that string and showed only its generic failure text. Normalize the first nonblank string among msg, message, and error, while continuing to omit the raw reason field.

The existing 0.4 turn lifecycle already treats an empty error frame as a failed turn with exit code 1. This change preserves that behavior and makes the server's available error text visible.

Verification

• npm run build
• node --test --test-isolation=none dist/test/stream.test.js dist/test/turn_lifecycle.test.js (38 passed)
• git diff --check

Synthetic regressions cover canonical field precedence, a blank msg with an error fallback, non-string error, hidden reason, sanitized terminal controls, a trailing done frame, and nonzero one-shot exit.

No package tag or publication is included.