[Extensibility] Add custom tool formatter API #552

Goal

Allow recipes to provide custom presentations for tool output.

Scope

• Register and unregister formatters by tool name.
• Require explicit opt-in when replacing a built-in formatter, with restoration on unregister.
• Preserve core ownership of borders, folds, streaming, errors, and renderer lifecycle.
• Document the contract and add focused coverage.

Out of scope

Replacing message-role or arbitrary message-part formatters.