Full prompt support + resource and tool content support improvments

[patvice]

Add Full Prompt Support to RubyLLM-MCP

Overview

This PR introduces comprehensive prompt support to the RubyLLM-MCP gem, allowing users to leverage predefined MCP prompts with arguments for consistent and powerful LLM interactions. This enhancement complements the existing tool and resource capabilities, making the gem a complete MCP integration solution.

Key Features Added

Prompt Management

• Prompt Discovery: List and access available prompts from MCP servers
• Argument Handling: Support for required and optional prompt arguments with validation
• Flexible Integration: Multiple ways to include prompts in conversations

Enhanced Chat Interface

• chat.ask_prompt(prompt, arguments: {}) - Direct prompt execution
• chat.with_prompt(prompt, arguments: {}) - Add prompts to conversation context
• prompt.include(chat, arguments: {}) - Direct prompt inclusion method

New Classes & Components

• RubyLLM::MCP::Prompt - Core prompt object with argument validation
• RubyLLM::MCP::Attachment - Message attachment handling
• RubyLLM::MCP::Content - Content type management
• RubyLLM::MCP::Completion - Completion data structures
• Prompt request handlers for list and call operations

Additional Improvements

Enhanced Tool Support

• Improved tool call handling to support formats beyond plain text
• Better response parsing and content extraction

Resource Enhancements

• Updated resource support to use proper content returns
• Better content handling and extraction