Move `Prompt` object instantiation from server to prompt manager

[ramjibc]

The Prompt() decorator in server.py is instantiating a Prompt object before adding it to the list of prompts. This is inconsistent with how the Tool() decorator works; it delegates tool instantiation to the tool manager.

Motivation and Context

To make the decorators consistent and create clear separation of concerns: I have moved prompt object instantiation from server.py to prompts/manager.py.

How Has This Been Tested?

I updated and ran all unit tests.

Breaking Changes

No. Users will not be impacted as the prompt() decorator's signature remains unchanged.

Types of changes

• [X] 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

• [X] I have read the MCP Documentation
• [X] My code follows the repository's style guidelines
• [X] New and existing tests pass locally
• [X] I have added appropriate error handling
• [ ] I have added or updated documentation as needed

Additional context

Separation of concerns: instantiation and managements of prompt objects should be done by the prompt manager, not the server.