fix(session): stop malformed tool-call loops

[Nomadcxx]

Issue for this PR

Fixes #31247

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Copilot Claude Opus 4.8 can return finish=tool-calls without sending a structured tool call. OpenCode treated that as a tool-loop turn and asked the provider again, which can lead to Copilot rejecting the next request as assistant prefill.

This PR stops the loop when that malformed response shape appears. It records a session APIError, marks the assistant turn as finish=error, publishes the existing session error event, and leaves valid tool-call turns unchanged.

How did you verify your code works?

cd packages/opencode
bun test --timeout 30000 test/session/prompt.test.ts -t "loop stops when finish is tool-calls without tool parts"
bun test --timeout 30000 test/session/prompt.test.ts

Observed:

targeted regression: 1 pass, 0 fail, 11 expect() calls
prompt.test.ts: 57 pass, 1 skip, 0 fail, 232 expect() calls

From repo root:

bun run typecheck
git diff --check HEAD~1..HEAD

Observed:

typecheck: 29 successful, 29 total
git diff --check: no output

Screenshots / recordings

Not applicable. This PR changes session-loop behavior, not UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR