update: add the system prompt class in mcp server

[syedmuneeb321]

Added SystemMessage class in MCP prompts so users can now return system prompts in their agents.

Motivation and Context

Before this, MCP only supported user and assistant prompts. There was no option for system prompts. Now, users can directly import SystemMessage and use it to set system-level instructions in their agents.

How Has This Been Tested?

Yes, tested in a local MCP agent app by creating a system prompt using SystemMessage. Verified that it works without errors and system prompts can be added successfully.

Breaking Changes

No breaking changes. This is a non-breaking new feature. Existing code will work as before.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the [MCP Documentation](https://modelcontextprotocol.io)
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

This was a simple but important addition to improve prompt control for agents. Now system instructions can be added directly using the new SystemMessage class.

[syedmuneeb321]

fix some issue

[Kludex]

Hi @syedmuneeb321 ,

Thanks for the PR, but this doesn't match the specification: https://modelcontextprotocol.io/specification/2025-06-18/server/prompts#promptmessage.