fix: emit null instead of empty string in JSON output

[julianschiavo]

• Changed DocumentationSearchDocument.contents and .title from String (defaulting nil to "") to String? so JSON output emits null for missing values
• Added custom encode(to:) to ensure nil optionals are always serialized as null keys rather than omitted
• Added regression tests in SearchJSONOutputTests verifying null emission and non-nil preservation

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Note

Low Risk
Low risk: only changes CLI JSON encoding of title/contents from empty strings to explicit null, plus adds focused regression tests.

Overview
Updates the search --json output to preserve missing fields as JSON null by making DocumentationSearchDocument.title/contents optional and encoding them explicitly (instead of coercing nil to empty strings or dropping keys).

Adds SearchJSONOutputTests to verify nil values serialize as null and non-nil values remain unchanged.

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