fix: Avoid unnecessary type casts in concat_ws

[neilconway]

Which issue does this PR close?

• Closes concat_ws does unnecessary type casts #20434.

Rationale for this change

1. concat_ws returned Utf8, regardless of the input types it was called with. If it was called with LargeUtf8, returning Utf8 might overflow. In general, functions like these should operate on all three string representations unless there is a compelling reason not to (e.g., this is how concat works).
2. simplify_concat_ws always constructed new literals with type Utf8. This lead to unnecessary casts when its inputs were of a different string type.

What changes are included in this PR?

• Support concat_ws return type matching its input types, following how concat does it.
• In simplify_concat_ws, construct literals with the right type, not always Utf8
• Refactor return_type for concat to be more readable
• Make StringViewArrayBuilder API more similar to the other string array builders, WRT null handling
• Add new unit and SLT tests
• Update test output for changed types

Are these changes tested?

Yes.

Are there any user-facing changes?

Yes: some queries involving concat_ws will now omit unnecessary cast operations, and the return type of concat_ws might be any of the three string types. Generally these changes should match user expectations better than the previous behavior.

[Omega359]

I did a quick look at the changes and nothing obvious jumped out at me. I'll try and find time to do a more extensive review if no one else beats me to it.

[neilconway]

@Omega359 Thank you!

[Omega359]

🤖 /home/bruce/gh_compare_branch_bench.sh Benchmark Script Running
Linux fedora 6.18.12-200.fc43.x86_64 #1 SMP PREEMPT_DYNAMIC Mon Feb 16 18:58:26 UTC 2026 x86_64 GNU/Linux
Comparing neilc/concat-ws-type-fixes (9c0b4f4) to c3f0807 diff
BENCH_NAME=concat_ws
BENCH_COMMAND=cargo bench --bench concat_ws
BENCH_FILTER=
BENCH_BRANCH_NAME=neilc_concat-ws-type-fixes
Results will be posted here when complete

[Omega359]

🤖: Benchmark completed

Details

group                                  main                                   neilc_concat-ws-type-fixes
-----                                  ----                                   --------------------------
concat_ws function/concat_ws/1024      1.00     11.1±0.08µs        ? ?/sec    1.16     12.9±2.43µs        ? ?/sec
concat_ws function/concat_ws/4096      1.00     44.1±0.88µs        ? ?/sec    1.02     44.8±1.95µs        ? ?/sec
concat_ws function/concat_ws/8192      1.00     88.5±3.98µs        ? ?/sec    1.02     90.4±3.22µs        ? ?/sec
concat_ws function/concat_ws/scalar    1.00     28.3±0.15µs        ? ?/sec    1.02     28.8±0.41µs        ? ?/sec

[Omega359]

I wonder if all this duplicated code could be eliminated with an approach similar to

datafusion/datafusion/functions-nested/src/string.rs

Line 792 in 2a08013

trait StringArrayBuilderType: ArrayBuilder {

?

[neilconway]

Yeah, I think that would make sense to do. I'm inclined to do it as a follow-up PR -- let me know if you'd prefer it as part of this PR.