fix: allow only public legacy stream errors

[AetherAI3]

Summary

The alternate SSE error field is not a general public-message contract: some historical media paths placed provider exception text there. Restrict the 0.4 client fallback introduced in #196 to two audited, fixed legacy public media messages. Continue to accept canonical msg and message strings. All other error values use the existing generic failed-turn message; reason remains omitted.

The client still exits nonzero on every error frame and ignores any trailing done frame.

Verification

• npm run build
• node --test --test-isolation=none dist/test/stream.test.js dist/test/turn_lifecycle.test.js (39 passed)
• git diff --check

Synthetic regressions cover both fixed legacy messages, canonical precedence, strict allowlist matching, a secret-looking arbitrary error with terminal controls, hidden reason, a trailing done frame, and one-shot exit code 1.

No package tag or publication is included.