feat: support binary and string types for concat UDFs

[theirix]

Which issue does this PR close?

• Closes Binary string (BYTEA, Binary) concatenation #12709.

Rationale for this change

While #21883 introduced binary argument support for the pipe operator, this PR targets three UDFs: concat, concat_ws, and Spark's concat to harmonise all their behaviour.

We decided to support only string+string and binary+binary and ban mixed operations, to match the behaviour of the majority of engines. Previously, mixed behaviour was allowed in #20787.

What changes are included in this PR?

• Added support for binary types (four variants)
• Disallowed mixed string/binary operations
• Fixed edge cases when binary->string type coercion overrode UDF rules, so mixed calls were allowed
• Refactored the three UDFs by extracting duplicate code into shared helpers to keep the logic centralised
• Refined concat_ws behavior for different separator types

The diff is quite large. Detailed code changes:

• Added a trait ConcatBuilder to abstract string/binary/view/array operations
• Abstracted StringArray/LargeStringArray into a generic ConcatGenericStringBuilder - less duplication
• Introduced mirrored builders for binary types in a new file binaries.rs
• Introduced more ColumnarValueRef variants to handle nullable and non-nullable binary types
• Extracted the ColumnarValue -> ColumnarValueRef builder to from_columnar_value - simplified call sites. Scalar code path stays mostly the same
• Switched from Signature::variadic to Signature::UserDefined to allow different argument types.Variadic required every argument (including binaries) to be coerced to the same string type, so the UDF cannot distinguish between binary and string inputs. It's a relatively uncommon - happy to discuss
• Simplified Spark's concat significantly by reusing the concat implementation, so it handles only Spark-specific null-handling
• Moved SLTs from Spark SLT (spark/concat.slt) to a generic SLT, so we test both generic and Spark-specific behaviour

Are these changes tested?

• Added more unit tests for previously uncovered major code paths
• Added more SLTs, especially for type coercion

Are there any user-facing changes?

No