[VL] Fix json_tuple rewrite producing incompatible JSON path in fallback scenarios

[Zouxxyy]

What changes are proposed in this pull request?

When json_tuple is rewritten to get_json_object calls by PullOutGenerateProjectHelper.pullOutPreProject, the generated JSON path used bare bracket notation $[key] (without quotes). This works fine in Velox/simdjson, but when the expression falls back to Spark JVM execution (e.g., get_json_object is blacklisted or validation fails), Spark's JsonPathParser rejects $[key] and returns NULL.

This PR changes the generated path format from $[key] to $['key'] (single-quoted bracket notation), which is accepted by both Velox/simdjson and Spark JVM's JsonPathParser.

Root cause:

• Velox's JsonPathNormalizer normalizes $['key'] → $[key] internally for simdjson, so both forms work.
• Spark JVM only accepts $.name or $['name'], and rejects $[name] (bare brackets without quotes), returning NULL directly.

How was this patch tested?

Added unit tests in MiscOperatorSuite covering:

• Basic single key extraction with fallback
• Dot-containing field names (e.g., a.b) — the core scenario for bracket notation
• Multiple keys extraction
• Non-existent keys returning null
• Mix of existing and non-existing keys
• NULL JSON input handling

Was this patch authored or co-authored using generative AI tooling?

Yes

Generated-by: Qoder

[Zouxxyy]

The failure ci should be unrelated, CC @lyy-pineapple @philo-he for a look thanks

[philo-he]

Thank you for the PR.
MiscOperatorSuite.scala generally holds some basic tests for expressions/operators. I would recommend to move the proposed test to a new test suite, named like JsonTuplePathRewriteSuite.scala. Thanks.

[philo-he]

LGTM. Thank you!