Add _meta support to registerPrompt

[chughtapan]

Prompts were the only primitive missing _meta passthrough. This adds _meta to RegisteredPrompt, _createRegisteredPrompt, registerPrompt, and the prompts/list handler, mirroring the existing tool pattern.

Motivation and Context

This improves protocol conformance in the SDK.

How Has This Been Tested?

New and existing tests pass

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

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Latest commit: 01a51f6

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