[C++] Migrate to `simdjson` in `from_string.cc` and tests

[taepper]

Describe the enhancement requested

from_string.cc should be migrated to simdjson.

Parent issue: #35460

Component(s)

C++

[KHARSHAVARDHAN-eng]

Hi!

I'd like to work on this issue if no one is currently working on it.

I'll investigate the existing simdjson migration pattern and submit a PR following the established implementation.

Thanks!

[Reranko05]

Hi! Just a note that this issue is part of the repository-wide RapidJSON → simdjson migration (GH-35460). I am working through the migration as a series of smaller PRs to avoid duplicate work.

[KHARSHAVARDHAN-eng]

Thanks for the clarification!

I wasn't aware this migration was already being coordinated through a series of smaller PRs.

I've already prepared PR #50644 for from_string.cc. If this overlaps with work that's already in progress, I'm happy to close or retarget the PR.

Otherwise, if from_string.cc is still available as part of the migration effort, I'm happy to continue updating the PR based on review feedback.

Thanks!