fix: map StructuredOutput finish to stop, not tool_calls

[djlongy]

Summary

Opencode ends response_format: json_schema turns on its internal StructuredOutput tool, so the finish string contains "tool". mapFinishReason previously mapped any finish containing "tool" to tool_calls even when the proxy had already flattened schema JSON into message.content and toolCalls was empty.

Clients using the Vercel AI SDK (generateText + Output.object / supportsStructuredOutputs: true) only parse structured output when the unified finish reason is stop. They otherwise throw AI_NoOutputGeneratedError despite valid JSON in message.content.

Fix

• mapFinishReason: stop mapping "tool" → tool_calls; return stop (callers already force tool_calls when toolCalls.length > 0).
• mapFinishReasonToAnthropic: same for "tool" → tool_use (callers force tool_use when tools present).
• Gemini already returned STOP for tool finishes — unchanged.
• Unit tests updated to match.

Real tool calling is unchanged: createChatCompletionResponse / streaming / Anthropic builders still set finish_reason: "tool_calls" / stop_reason: "tool_use" from toolCalls.length.

Minimal repro (before)

curl -sS -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
  -d '{
    "model":"openai/gpt-5.5",
    "messages":[{"role":"user","content":"Return {\"ok\":true} only."}],
    "response_format":{
      "type":"json_schema",
      "json_schema":{
        "name":"probe",
        "schema":{"type":"object","properties":{"ok":{"type":"boolean"}},"required":["ok"]}
      }
    }
  }' http://127.0.0.1:4010/v1/chat/completions

Field	Before	After
finish_reason	tool_calls	stop
message.content	{"ok":true} JSON	same
message.tool_calls	absent / empty	absent

Test plan

• npm test — 242 pass locally
• Confirm chat/completions with caller tools still returns finish_reason=tool_calls + populated tool_calls
• Confirm json_schema returns finish_reason=stop + JSON content

Notes

• Branched from main (current line including Spurious 'failed to load plugin' error on opencode 1.18.x (cosmetic — proxy still works); one-line V1 default export silences it #86 default export). CONTRIBUTING mentions dev, but dev is behind main and recent fixes (Spurious 'failed to load plugin' error on opencode 1.18.x (cosmetic — proxy still works); one-line V1 default export silences it #86/fix(plugin): add V1 default export descriptor to stop legacy loader probing every helper #89) landed on main.
• Happy to retarget dev if preferred.