docs: add prompt client/server example to address #498

[cubaseuser123]

Added a runnable client/server snippet pair demonstrating the end-to-end flow for MCP prompts.

Motivation and Context

Following up on #498, it seemed like a good idea to add a full end-to-end example for prompts. I put together a minimal client/server pair that shows exactly how a client discovers @mcp.prompt() definitions and retrieves their messages.

This PR adds a minimal, runnable pair to clarify the flow:

• examples/snippets/servers/prompt_server.py — defines two prompts covering both return styles: a plain str and a list[Message] multi-turn thread. Registered in the snippets server dispatcher so it runs via uv run server prompt_server.
• examples/snippets/clients/prompt_client.py — connects via stdio, calls list_prompts() to discover what's available (with arguments), then calls get_prompt() for each and prints the resulting TextContent. Registered as uv run prompt-client.

How Has This Been Tested?

• Ran uv run prompt-client locally to verify the full flow works end-to-end against the server.
• Both prompts list correctly and return the expected TextContent message boundaries.
• Ran uv run ruff check and uv run pyright on the new files (0 errors).
• Ran the full test suite (pytest tests/server/mcpserver/test_server.py), all 89 tests pass cleanly.

Breaking Changes

None

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
• 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

Let me know if there's a preferred location for this, or if the docs site needs a dedicated Markdown page walking through the prompt flow as well. Happy to add that. Fixes #498.