Add MCP prompt SDK support for rich return types

[liliankasem]

Issue describing the changes in this PR

resolves #issue_for_this_pr

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• I have added all required tests (Unit tests, E2E tests)

Additional information

Add FunctionsMcpPromptResultMiddleware to the Worker.Extensions.Mcp.Sdk
package, enabling prompt functions to return rich types (GetPromptResult,
PromptMessage, IList) instead of only plain strings.

The middleware follows the same pattern as FunctionsMcpToolResultMiddleware:

• Intercepts prompt function results in the worker pipeline
• Serializes them into McpPromptResult wrapper format
• Host-side PromptReturnValueBinder deserializes and processes the wrapper

Supported return types:

• GetPromptResult: serialized directly with description and messages
• PromptMessage: wrapped in a single-element list
• IList: serialized as message array
• string/object: wrapped as text in a user PromptMessage (default)