[0xdeafbeef]

Which issue does this PR close?

Found while fixing the SQL unparser's handling of SIMILAR TO in #26831; no separate issue.

Rationale for this change

After a logical plan round trip through datafusion-proto, a case-insensitive SimilarTo matches case-sensitively.
SimilarToNode had no field for case_insensitive.
The encoder ignored the flag, and the decoder passed false.
The physical planner runs an Expr::SimilarTo with case_insensitive: true as a case-insensitive regex match (RegexIMatch / RegexNotIMatch).
The SQL planner always creates Expr::SimilarTo with case_insensitive: false, so only expressions built in Rust carry true.

What changes are included in this PR?

SimilarToNode gets bool case_insensitive = 5, which to_proto writes and from_proto reads.
prost.rs and pbjson.rs are regenerated with the datafusion/proto-models/gen generator, and the generated diff touches only SimilarToNode.
The pbjson serializer writes caseInsensitive only when it is true, so the JSON of a case-sensitive SimilarTo does not change.
A payload without the field decodes as false, the same result as before this PR.

The other option is a separate message, as LIKE and ILIKE use LikeNode and ILikeNode.

What is the testing strategy for this PR?

roundtrip_similar_to in datafusion/proto/tests/cases/roundtrip_logical_plan.rs takes a case_insensitive parameter and gets two cases with true: similar_to(false, None, true) and similar_to(true, Some('X'), true).
Without the fix the test fails on the first of them: the decoded expression has case_insensitive: false.

I ran these checks locally, not in CI:

• cargo test -p datafusion-proto --features json,avro: lib tests, doc-tests and proto_integration pass, except seven parquet source tests that need the parquet-testing submodule data, which the local checkout did not have.
  The same seven fail without this change.
• cargo fmt --all -- --check, cargo clippy -p datafusion-proto -p datafusion-proto-models --all-targets -- -D warnings with and without --all-features, typos on the changed files, and prettier on the upgrade guide pass.

Are there any user-facing changes?

The generated SimilarToNode struct has a new public field, so code that builds it with a struct literal must set case_insensitive.
docs/source/library-user-guide/upgrading/56.0.0.md gets a section with a before/after struct literal.
This change needs the api change label.