fix(sparksql): validate json_object_keys input

[SC123667]

What problem does this PR solve?

Issue Number: close #742

json_object_keys used simdjson's On-Demand API without checking lazy parse errors while reading the root type, iterating fields, or unescaping keys. Malformed JSON could therefore raise a parser exception instead of returning NULL, and trailing content could be accepted.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 🚀 Performance improvement (optimization)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• 🔨 Refactoring (no logic changes)
• 🔧 Build/CI or Infrastructure changes
• 📝 Documentation only

Description

• Check the lazy root-type parse result before accessing it.
• Return NULL for malformed object fields and invalid escaped keys.
• Reject valid objects followed by trailing non-whitespace content.
• Cover the issue examples, nested objects, non-object inputs, malformed structures, invalid literals, escaped keys, and trailing content.

Performance Impact

• No Impact: The change adds lightweight error checks to parser operations already performed by this function.
• Positive Impact: I have run benchmarks.
• Negative Impact: Explained below (e.g., trade-off for correctness).

Release Note

Release Note:
- Fixed SparkSQL json_object_keys to return NULL for malformed JSON instead of surfacing parser errors or accepting trailing content.

Checklist (For Author)

• I have added/updated unit tests (ctest).
• I have verified the code with local build (Release/Debug).
• I have run clang-format / linters.
• (Optional) I have run Sanitizers (ASAN/TSAN) locally for complex C++ changes.
• No need to test or manual test.

Breaking Changes

• No
• Yes (Description: ...)