fix(checks): accept parsed string JSON values

[harsh21234i]

Closes #2511

Summary

• allow JsonValid to accept non-empty plain Python strings as already parsed JSON scalar values
• keep parsing serialized JSON-looking strings such as objects, arrays, quoted strings, numbers, booleans, and null
• keep empty strings and malformed JSON-looking strings failing as invalid JSON
• add coverage for plain string outputs and schema validation against plain strings

Testing

• uv run --no-sync -m pytest -q libs/giskard-checks/tests/builtin/test_json_valid.py
• uv run --no-sync ruff check libs/giskard-checks/src/giskard/checks/builtin/json_valid.py libs/giskard-checks/tests/builtin/test_json_valid.py

Verification: 23 passed, ruff passed.

[gemini-code-assist]

Code Review

This pull request updates the JSON validation logic to accept plain strings as valid inputs instead of attempting to parse them as JSON and failing. It introduces a helper method _looks_like_serialized_json to identify if a string is serialized JSON. The reviewer provided critical feedback regarding potential bugs in this helper: whitespace-only strings can bypass the empty string check, and the prefix-matching logic is too loose, which would incorrectly classify plain strings starting with JSON-like prefixes (e.g., "truth", "nullify") as serialized JSON and cause parsing errors. Robust code suggestions were provided to address both issues.

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[harsh21234i]

hey @kevinmessiaen @davidberenstein1957 can someone look into this?

[kevinmessiaen]

Fixed by #2575