todo: migrate tool output to structured JSON schemas

[dgageot]

Replace plain-text string output with proper JSON-structured responses for all todo tools. Each tool now has a typed OutputSchema matching its actual output:

• create_todo: returns a Todo object
• create_todos: returns CreateTodosOutput with created []Todo
• update_todos: returns UpdateTodosOutput with updated/not_found
• list_todos: returns ListTodosOutput with todos []Todo

Other improvements

• Extract addTodo helper to deduplicate todo creation logic
• Extract jsonResult helper to deduplicate JSON marshaling + ToolCallResult construction
• Reuse Todo type as create_todo output (eliminates redundant CreateTodoOutput struct)
• Fix jsonschema descriptions for consistency ("New status" → "Status of the todo item", add missing space)
• Refactor updateTodos error path to reuse jsonResult instead of duplicating marshal logic
• Add requireMeta test helper to reduce assertion boilerplate

[docker-agent]

🔍 cagent Review

Assessment: 🟢 APPROVE

The refactoring successfully migrates todo tool output to structured JSON schemas with proper error handling and helper function extraction. No critical or high-severity bugs were found.

One minor design inconsistency was noted for consideration:

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Optional Improvement

While not blocking, there's a pattern inconsistency in the output structure design:

createTodo returns bare Todo while other tools use wrapper types

• createTodo returns: Todo directly
• createTodos returns: CreateTodosOutput{Created: []Todo}
• updateTodos returns: UpdateTodosOutput{...}
• listTodos returns: ListTodosOutput{...}

Since this PR explicitly aims for "proper JSON-structured responses," consider wrapping createTodo's output in a dedicated type (e.g., CreateTodoOutput{Todo: Todo}) for consistency with the other tools.

The current implementation works correctly and tests pass—this is purely about maintaining a consistent pattern across all tool outputs.

[docker-agent]

Optional Improvement: Output Structure Consistency

The createTodo function returns a bare Todo object, while all other tools wrap their outputs in dedicated types:

• ✅ createTodos → CreateTodosOutput{Created: []Todo}
• ✅ updateTodos → UpdateTodosOutput{...}
• ✅ listTodos → ListTodosOutput{...}
• ⚠️ createTodo → Todo (bare object)

Since this PR explicitly refactors for "proper JSON-structured responses," consider wrapping this output in a dedicated type (e.g., CreateTodoOutput{Todo: Todo}) for consistency.

Not blocking—the current implementation works correctly.