chat: "an unknown error has occurred" toast shown on first message

Summary

When starting opencode2 and sending the first chat message, a toast in the
upper-right corner displays "an unknown error has occurred". The error text
is unhelpful and no underlying cause is surfaced to the user.

Environment

• opencode version: opencode2 v0.0.0-beta-17927
• OS: Darwin 25.6.0 (darwin arm64)
• Terminal: TERM_PROGRAM=WarpTerminal, TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/zsh
• Install/channel: beta
• Active plugins: None found in config

Reproduction

1. Launch opencode2 in a terminal (WarpTerminal).
2. Start a fresh chat session.
3. Send the first message to a cloud API provider.
4. Observe the toast in the upper-right corner: "an unknown error has occurred".

Expected Behavior

No error toast should appear on the first message, or if a real error occurs
it should surface an actionable message (provider error, status, logs) instead
of the generic "an unknown error has occurred".

Actual Behavior

A toast reading "an unknown error has occurred" appears in the upper-right
corner of the TUI every time the first message is sent. (Assumption: the chat
still answers normally despite the toast — please correct if the message fails
to send.) No detailed error, log line, or stack trace is shown.

Additional Context

• Reproducible: every time (consistent).
• Provider: cloud API provider (e.g. OpenAI/Anthropic/Google).
• Only the toast text is visible; no additional error detail or logs captured.
• No recent config change, plugin install, or workspace switch reported.
• No workaround attempted yet.