fix(session): stop prompt loop after model completion

[luyanhexay]

Issue for this PR

Closes #17982
Refs #18096
Refs #11153

Type of change

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

What does this PR do?

This PR fixes a session loop termination bug after model completion.

• stop prompt-loop continuation when the model has already completed and no tool activity remains
• treat finish=unknown as terminal only for non-tool responses
• keep json-schema/tool-call paths unchanged

Why this works:

• terminal completion state is handled explicitly so the loop cannot emit extra assistant messages in the same turn
• tool-related paths are excluded from the terminal shortcut, preserving existing tool execution flow

How did you verify your code works?

• bun test ./packages/opencode/test/session/session.test.ts (3 passed, 0 failed)
• repeated local reproduction with anthropic/claude-sonnet-4-6 no longer produced multi-assistant emissions after one user ping

Screenshots / recordings

N/A (non-UI change)

Checklist

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