feat(tools): add function and tool calling helpers

[leehack]

Feature description

Add first-party helpers for function/tool calling so apps can define tools, render model-family-specific prompts/templates, parse tool calls, and dispatch results safely.

Motivation

Tool calling is now a core local-agent capability. llamadart can provide a cleaner developer experience by standardizing common pieces instead of leaving every app to hand-roll templates and parsers.

Proposed scope

• Define a small Dart representation for tools/functions, parameters, and tool-call results.
• Render model-family-specific tool-use templates where feasible.
• Parse tool-call output robustly, including whitespace and streaming edge cases.
• Provide validation and safe error reporting for malformed calls.
• Include an example that integrates with mcp_dart tools as a local-agent story.

Acceptance criteria

• Docs include at least one complete local tool-calling example.
• Tests cover parser behavior across well-formed and malformed outputs.
• The API does not require a specific UI framework.