fix(opencode): stop sending tools when tool_call is false

[tobwen]

Issue for this PR

Closes#19966
Closes#35432

Type of change

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

What does this PR do?

tool_call: false in model config was stored in capabilities.toolcall but never checked in the prompt loop. SessionTools.resolve() ran unconditionally, tools were sent in every LLM request with tool_choice: "auto". Providers that do not support tool calling (e.g. local Ollama models, morphllm without special flags) returned 400.

Three changes in packages/opencode/src/session/prompt.ts:

1. Guard SessionTools.resolve() behind model.capabilities.toolcall - skip tools entirely when false.
2. Set toolChoice: "none" when toolcall is false (the default undefined becomes "auto" at the provider).
3. When format.type === "json_schema" and toolcall is false, fail fast with a clear StructuredOutputError instead of pushing a misleading system prompt and making a doomed LLM call.

How did you verify your code works?

• Read the AI SDK source: prepareToolsAndToolChoice drops both empty tools and toolChoice before the HTTP body is serialized, so no tools or tool_choice key reaches the provider when toolcalling is disabled.
• bun typecheck passes.
• Existing prompt tests pass. Added a regression test under noLLMServer.instance that verifies the loop does not crash with tool_call: false.
• Built a single binary and ran the smoke test.

Screenshots / recordings

Not a UI change.

Checklist

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

[tobwen]

As the discussion appeared on Discord. This config setting is clearly defined under model section in config.json-specs:

From a semantic perspective, this is also the correct place. It is defined here that the model does not support tools, not that the user does not want to have tools in an agent.

[github-actions]

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