fix(core): return content-only Code Mode results

[opencode-agent]

Summary

• promote text content into the Code Mode return value when a tool has no declared structured output
• keep declared structured outputs unchanged and continue collecting file content separately
• cover content-only Promise plugin and MCP tool execution through Code Mode

Fixes#41949

Testing

• bun test test/plugin/promise.test.ts test/mcp.test.ts
• bun typecheck
• bun x prettier --check packages/core/src/codemode/tool.ts packages/core/test/plugin/promise.test.ts packages/core/test/mcp.test.ts

Requested by: @rekram1-node (Aiden Cline via Slack)