fix(opencode): honor agent tool_choice config

[ozpool]

Issue for this PR

Closes #32465

Type of change

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

What does this PR do?

Setting tool_choice on an agent in opencode.json had no effect — the key was accepted without error and silently dropped, so the provider always received tool_choice: "auto". For models that don't reliably emit tool calls under auto, that makes agentic runs impossible (the model never gets asked to commit to a tool call).

The llm layer already plumbs toolChoice end to end (session/llm.ts → native request/runtime → per-provider lowering); the only missing link was config → request. The prompt builder hard-coded toolChoice to undefined for normal turns (it was only ever set to "required" for json_schema output).

This wires the existing capability up:

• adds tool_choice ("auto" | "required" | "none") to the agent config schema and registers it as a known key so it stays a top-level field instead of being swept into options,
• adds the matching toolChoice field to the loaded agent info and maps tool_choice → toolChoice during agent loading (next to temperature / top_p),
• uses the agent's configured toolChoice when building the request.

json_schema output still forces "required". When tool_choice is unset, toolChoice stays undefined and behavior is unchanged.

How did you verify your code works?

Added a schema test asserting tool_choice is preserved as a top-level field (not moved into options) and that an unknown value is rejected. Existing agent-loading suite still passes.

bun test packages/core/test/config/agent-v1.test.ts   # 2 pass
bun test packages/opencode/test/agent/agent.test.ts    # 43 pass
bun run typecheck                                      # clean (core + opencode)

Screenshots / recordings

Not a UI change.

Checklist

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

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found:

• PR fix(agent): apply configured tool_choice to model requests #32518: fix(agent): apply configured tool_choice to model requests
  • fix(agent): apply configured tool_choice to model requests #32518

Why it's related: PR #32518 addresses the exact same issue—applying configured tool_choice to model requests. Both PRs appear to solve the problem of honoring tool_choice settings from agent config in opencode.json. This is likely a duplicate or overlapping fix for issue #32465.