fix: force tool calls when outputFormat is json_schema

When structured output is requested, the model must call the
StructuredOutput tool instead of responding with plain text.

Changes:
- Add toolChoice parameter to LLM.StreamInput
- Pass toolChoice: "required" to streamText when outputFormat is json_schema
- This forces the model to call a tool, ensuring structured output is captured

packages/opencode/src/session/llm.ts

@@ -28,6 +28,7 @@ export namespace LLM {
     small?: boolean
     tools: Record<string, Tool>
     retries?: number
+    toolChoice?: "auto" | "required" | "none"
   }
 
   export type StreamOutput = StreamTextResult<ToolSet, unknown>
@@ -146,6 +147,7 @@ export namespace LLM {
       providerOptions: ProviderTransform.providerOptions(input.model, params.options),
       activeTools: Object.keys(tools).filter((x) => x !== "invalid"),
       tools,
+      toolChoice: input.toolChoice,
       maxOutputTokens,
       abortSignal: input.abort,
       headers: {

packages/opencode/src/session/prompt.ts

@@ -573,6 +573,7 @@ export namespace SessionPrompt {
         ],
         tools,
         model,
+        toolChoice: outputFormat.type === "json_schema" ? "required" : undefined,
       })
 
       // Handle structured output logic