Preserve field metadata through physical CASE expressions

[osipovartem]

Summary

Port apache/datafusion#26158 to our pinned DF55 fork. Physical CASE preserves agreed Arrow field metadata, including nested CASE and casts. The planner retains the logical result field when physical lowering loses metadata context, and physical-expression protobuf round-trips that field. The common unmarked Column/Literal path is unchanged.

The matching logical CASE change was merged into embucket-sync-df55.0.0 via #126. This branch is rebased onto that base. The Rustice integration uses a separately pinned branch containing both changes.

Validation

• cargo +stable test -p datafusion-physical-expr --features proto --lib --quiet: 1,688 passed, 2 ignored on this branch.
• Regression tests cover matching/conflicting metadata, NULL and typed-NULL branches, nested CASE and casts, alias-lowered marked UDFs, scalar variables, marked NOT, child rewrites, and protobuf encode/decode.
• The combined metadata.slt SQLLogicTest passed on the rebased branch, including direct and projected CASE metadata checks.
• Formatting, diff checks, and focused Clippy passed. Independent read-only review approved the rebased branch at 54db65b1c with no blockers.
• Nested CASE planning at depths 16/32/64/128 measured approximately 0.4/1.3/4.5/17.1 ms in the shared implementation. There is no per-row execution overhead; the common unmarked fast path remains unchanged.

Upstream: apache#26158