[fix](fe) Preserve inferred JSON column name casing

[0AyanamiRei]

What problem does this PR solve?

Issue Number: None

Related PR: None

Problem Summary: JSON schema inference lowercased field names before creating the table-valued function schema, while JSON readers match object keys case-sensitively. Uppercase keys therefore appeared as queryable lowercase columns but were silently materialized as NULL. Preserve the inferred key casing for JSON only, while retaining case-insensitive duplicate detection and the existing lowercase behavior for other file formats.

Release note

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

Check List (For Author)

• Test: Regression test and manual tests
  • Regression test: test_json_uppercase_keys (raw and jsonpaths control)
  • Manual test: File Scanner V2, legacy scanner, and http_stream returned the two expected rows
  • Manual compatibility test: CSV inferred column names remained lowercase
• Behavior changed: Yes. Inferred JSON field names preserve source casing.
• Does this need documentation: No

[hello-stephen]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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