[sollhui]

What problem does this PR solve?

Issue Number: None

Related PR: #67411

Problem Summary: Backport #67411 to branch-4.2. JSON schema inference lowercased field names while JSON readers match object keys case-sensitively, causing mixed-case and uppercase keys to return NULL. Preserve inferred JSON key casing, retain case-insensitive duplicate detection, and keep existing lowercase behavior for other file formats.

The conflict resolution also preserves branch-4.2's existing Lance column name casing behavior.

Release note

JSON table-valued functions now read inferred mixed-case and uppercase keys without silently producing NULL values.

Check List (For Author)

• Test: Compilation and tests skipped as requested. The original PR's regression test and fixture are included in this backport. CI requested with run buildall.
• Behavior changed: Yes. Inferred JSON field names preserve source casing.
• Does this need documentation: No.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[sollhui]

run buildall

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 87.50% (7/8) 🎉
Increment coverage report
Complete coverage report

[yiguolei]

skip buildall

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

PR approved by at least one committer and no changes requested.