fix: clarify StringViewBuilder.UnmarshalOne expected type

[fallintoplace]

Summary

Problem

StringViewBuilder.UnmarshalOne reports a json.UnmarshalTypeError with an expected type of []byte{} for unsupported token types.

Change

• Update StringViewBuilder.UnmarshalOne to report reflect.TypeOf("") when an unsupported JSON token is encountered.
• Add regression test TestStringViewBuilderUnmarshalOneWrongType in arrow/array/string_test.go.

Impact

• Aligns error diagnostics with StringBuilder behavior.
• Makes JSON import debugging clearer for users expecting string inputs.

Testing

• Added a focused regression test that asserts the reported expected type is string for unsupported JSON token input.
• No full test run in this PR (single-path, message-type assertion).

[zeroshade]

The production change (reflect.TypeOf([]byte{}) -> reflect.TypeOf(string(""))) is right — a StringViewBuilder should report string. But the test doesn't compile:

arrow/array/string_test.go:914:27: cannot use dec (variable of type *encoding/json.Decoder)
  as *github.com/apache/arrow-go/v18/internal/json.Decoder value in argument to bldr.UnmarshalOne

UnmarshalOne takes arrow's vendored *internal/json.Decoder, not the stdlib one. Import github.com/apache/arrow-go/v18/internal/json (e.g. import json "github.com/apache/arrow-go/v18/internal/json") in the test instead of encoding/json. With that it should pass. Verified against the PR head with go test.

[zeroshade]

@fallintoplace nudge — the production change is correct; only the test is broken: it uses the stdlib encoding/json.Decoder, but UnmarshalOne takes arrow's internal/json.Decoder, so it won't compile. Swap the import and it should pass. Happy to re-review.