Case-insensitive field matching in struct casting

## Problem

This is a **design discussion** issue regarding whether DataFusion should adopt **case-insensitive field matching** when [casting between structs.](https://github.com/apache/datafusion/pull/19674)

### Current Behavior
DataFusion uses **case-sensitive** field name matching. For example:
- Field `x` and field `X` are treated as different fields
- A cast from `struct` to `struct` would fail (no name overlap)

### DuckDB Behavior
DuckDB uses **case-insensitive** field name matching:
- Field `x` and field `X` are treated as the same field
- The same cast would succeed, with fields matched case-insensitively

## Motivation for Case-Insensitive Matching

**Pros:**
- ✅ **Aligns with DuckDB** — improves compatibility with a major SQL database
- ✅ **More forgiving** — handles common casing variations (e.g., JSON sources with inconsistent field names)
- ✅ **Follows SQL conventions** — SQL generally treats identifiers as case-insensitive
- ✅ **User-friendly** — reduces friction when working with data from different sources

## Arguments for Keeping Case-Sensitive Matching

**Pros:**
- ✅ **Arrow foundation** — DataFusion is built on Apache Arrow, which is case-sensitive:
- ✅ **Language consistency** — matches Rust and JSON conventions (case-sensitive)
- ✅ **Prevents ambiguity** — avoids edge cases where source has both `x` and `X` (rare but possible)
- ✅ **Predictable behavior** — case-sensitive matching is more explicit and easier to reason about in programmatic contexts

## Question

**Should DataFusion follow SQL's case-insensitivity or remain aligned with Arrow's case-sensitive semantics?**

## Next Steps

This issue is intended to **surface the design question** and gather community feedback.