Feature: Add collection methods for dynamic Agent configuration

[sumitaryal]

Summary

This PR introduces new methods in the Agent class to enable the dynamic addition of various components (tools, handoffs, input guardrails, and output guardrails) in a scalable and type-safe manner.

Changes

• _add_item: A generic helper function to perform type checking and avoid duplicate additions.
• _ensure_iterable: A static method to ensure inputs are processed as iterables (handling single values and collections uniformly).
• add_tools: Allows adding one or multiple tools, using valid tool types from the union.
• add_handoffs: Supports the addition of one or multiple handoffs.
• add_input_guardrails: Enables adding one or more input guardrails.
• add_output_guardrails: Supports adding one or multiple output guardrails.

Motivation

By splitting the addition functionality into small, reusable methods, we improve the maintainability and extensibility of the Agent class. These changes also enforce type safety and prevent duplicates, ensuring robust runtime behavior.

Testing

• Verified that each method adds the correct items without duplicates.
• Confirmed type-checking is enforced correctly for each collection.

[rm-openai]

I'd prefer people just add directly by modifying the agent.

agent.tools.append(...)
agent.guardrails.extend(...)

etc