feat(schema): add utility methods for Document

[voocel]

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• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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(Optional) Which issue(s) this PR fixes:

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• Add Clone() method for creating document copies
• Add IsEmpty() method to check if document has content
• Add HasMetaData() method to check metadata existence
• Add ClearMetaData() method to remove all metadata
• Add WithMetaData()/GetMetaData() methods for custom metadata
• All methods support method chaining where appropriate
• Added comprehensive test coverage with 48 test assertions

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