refactor: introduce IncludeContents type

[aurelsandu]

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[gemini-code-assist]

Code Review

This pull request replaces raw string usage for the IncludeContents field with a dedicated internal type and adds JSON tags to the State struct. The review feedback highlights an inconsistency in the naming and value of the IncludeContentsAll constant, suggesting it be renamed to IncludeContentsDefault with the value 'default' to match existing logic and public APIs.