Preserve UDF source spans through JSON rewrites

[osipovartem]

Summary

• Preserve diagnostic spans when JSON get calls are flattened or rewritten for typed casts.
• Pin the Embucket DataFusion revision for standalone fork builds; the parent workspace patch remains authoritative when consumed by Rustice.
• Add focused tests for both rewrite paths.

Verification

• cargo +1.95.0 check --offline --all-targets
• cargo +1.95.0 test --offline --lib rewrite::tests (2 passed)
• Independent read-only review: approved, no blocking findings

This is the companion API update for Embucket/datafusion#150 and Rustice UDF diagnostic positions.