[C++] Simplify ObjectParser simdjson result handling

[Reranko05]

Describe the enhancement requested

As a follow-up to #50672, simplify ObjectParser by resolving simdjson_result<sj::field> earlier and reusing helper utilities from #50653 where appropriate, based on the review discussion. (Wait until #50672 is merged.)

Related review comments:

• Comment#1
• Comment#2
• Comment#3

Related PRs:

• GH-50654: [C++] Support simdjson without exceptions #50672
• GH-50627: [C++] Migrate from_string.cc to simdjson #50653

Component(s)

C++