fix(session): stop prompt loop after model completion

[luyanhexay]

Issue for this PR

Closes #17982
Refs #18096
Refs #11153

What this PR changes

• stops the session prompt loop when the model has already completed and there is no tool activity
• treats finish=unknown as terminal only for non-tool responses to prevent extra assistant-message iterations
• preserves existing json-schema and tool-call handling paths

Why this change is needed

A completed model response could still continue into another prompt-loop iteration, creating duplicate assistant outputs in one user turn. This patch makes loop termination explicit for that terminal state.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

How this was tested

• 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 a single ping

Checklist

• I linked an existing issue
• I tested locally
• This PR stays focused on one bug fix area

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