[C++] Address post-merge review comments in PR exposing {Array,...}FromJSON helpers in public API

[amoeba]

Describe the enhancement requested

#46180 was just merged to resolve #45908 and @pitrou left three comments after it was merged. Comments linked below:

• GH-45908: [C++][Docs] Rename and expose basic {Array,...}FromJSON helpers as public APIs #46180 (comment)
• GH-45908: [C++][Docs] Rename and expose basic {Array,...}FromJSON helpers as public APIs #46180 (comment)
• GH-45908: [C++][Docs] Rename and expose basic {Array,...}FromJSON helpers as public APIs #46180 (comment)

I'll reply to each in their place, update this issue with what changes we decided on, and file a PR to resolve them.

Component(s)

C++

[amoeba]

I've dropped comments in their respective locations in the above links. I may split this up into three PRs but currently have a draft up at #46447.

[amoeba]

I put up three PRs to fix each of the comments,

• GH-46439: [C++] Remove unneeded namespace prefix in test_util_internal.h #46695
• GH-46439: [C++] Use result pattern for all FromJSONString Helpers #46696
• GH-46439: [C++] Rename internal Converter class in from_string.cc #46697

[kou]

Issue resolved by pull request 46695
#46695