feat: SubAgent

[ishaksebsib]

Related Issue

#108

Description

This PR introduces subagents as a first-class capability in LanguageModelRequestBuilder.

A subagent can now be defined directly from the request builder's dedicated subagent path; .build() produces a Tool instead of a LanguageModelRequest, while normal request building remains unchanged.

The new API makes subagent creation explicit and type-safe:

• normal requests continue to use prompt() or messages()
• subagents are created with subagent(name, description)
• once in subagent mode, conversation input methods are no longer available
• The delegated task is supplied at tool-call time through the subagent input schema

Example

let subagent = LanguageModelRequest::builder()
    .model(model)
    .subagent("explore-agent", "Explore the codebase to have a deeper understanding of the project.")
    .system("You are a research agent. Summarize your findings in your final response.")
    .with_tool(readFileTool())
    .with_tool(searchTool())
    .build(); // -> Tool

let main_agent = LanguageModelRequest::builder()
    .model(model)
    .system("You are a helpful assistant with access to tools.")
    .prompt("What is the secret code for lets")
    .with_tool(subagent) // pass the subagent to parent
    .build()
    .generate_text()
    .await?;