feat: Implement get_tool_call_output method in RunResultBase and update doc

[hieutrtr]

Purpose:
The get_tool_call_output method provides a convenient way to retrieve the output of a specific tool call by its name. This is particularly useful when:

1. Working with multiple tool calls in an agent interaction
2. Need to access specific tool results for post-processing or validation
3. Want to check if a particular tool was called and what its output was

Problem It Solves:
Without this method, developers would need to:

1. Manually iterate through the new_items list
2. Match tool calls with their corresponding outputs using call_id
3. Handle potential errors and edge cases themselves
   This leads to repetitive code and potential errors. The method encapsulates this common operation in a clean, reusable way.