Fix conversation stream stalls

[TCP404]

Summary

• Finalize silent agent streams with a timeout error so the UI stops loading instead of spinning forever.
• Surface agent send failures through the stream relay immediately.
• Serialize bun path env override tests to avoid workspace test flakiness.

Verification

• cargo fmt --all -- --check
• cargo test -p aionui-runtime --lib
• cargo test -p aionui-conversation
• cargo clippy -p aionui-conversation -p aionui-runtime -- -D warnings
• cargo test --workspace
• just push -u origin fix/conversation-stream-timeout