Session silently stops on empty LLM response (finish: unknown, 0 tokens)

Description

When the model provider returns an empty completion (0 tokens, no text/reasoning/tool output, finish reason maps to unknown), opencode treats it as a normal completed turn:

• the session loop exits silently (packages/opencode/src/session/prompt.ts breaks on any finish that isn't tool-calls)
• no error is surfaced to the user, no retry happens
• the conversation appears to "suddenly stop" mid-task

Observed with the opencode-go gateway (deepseek-v4-flash) returning an empty response: the session went idle with the last assistant message recorded as finish: unknown with 0 tokens.

Repro

• Provider returns an SSE stream with no content/tool deltas and an unknown finish reason.
• After processing, the loop exits and the session goes idle with no error.

Expected

An empty response should be retried (or surfaced as an error), not silently treated as a completed turn.

Related: #40531 (upstream PR with the same fix, currently conflicting with dev).