Add JSON serialization and deserialization for ToolSpecification

Add JSON serialization and deserialization for ToolSpecification: a task in LegoFlow-SWE (Harbor dataset). Currently, ToolSpecification provides no way to serialize an instance to a JSON string or deserialize JSON back into an object. This prevents users from persisting tool definitions externally…

The task

Currently, `ToolSpecification` provides no way to serialize an instance to a JSON string or deserialize JSON back into an object. This prevents users from persisting tool definitions externally (e.g., in a database or configuration file) and later rehydrating them.

Part of Lego-X/LegoFlow-SWE.