fix: handle scalar content values in JSONConverter

[Cha-Imaa]

Related Issues

• fixes JSONConverter fails on numeric and boolean content_key values #12953

Proposed Changes

JSONConverter currently passes numeric and boolean content_key values directly to Document, which only accepts string or None content.

This change converts non-string scalar content_key values to strings before creating the Document, while preserving existing behavior for strings and None. Objects and lists continue to be skipped.

How did you test it?

• Reproduced the issue with a numeric content_key value (123), which previously raised ValueError.
• Added regression coverage for numeric and boolean scalar values.
• pytest test/components/converters/test_json.py → 28 passed.
• ruff check haystack/components/converters/json.py test/components/converters/test_json.py → passed.
• ruff format --check haystack/components/converters/json.py test/components/converters/test_json.py → passed.

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