Reject partial parses in stou32

[Daksha1611]

🛠 Summary

Fixes #4554.

stou32() is the only converter in stringutils that does not verify the whole string was consumed. std::stoul stops at the first character it cannot use and still reports success, so stou32("12abc") returned 12, stou32("3.9") returned 3 and stou32("0x10") returned 0. Its siblings all guard against this by passing &idx and comparing against the length — stou64 at :143-147, stoi32 at :161-165, stof at :203-206. This adds the same check.

Two call sites are affected. grpcservermodule.cpp:79 reads the GRPC_SERVERS environment variable, so GRPC_SERVERS=4x silently started 4 servers instead of being rejected and falling back to config.grpcWorkers(). s2t_servable.cpp:81 uses it as a fallback for the transcription temperature field; that endpoint is also addressed in #4553, and either change alone fixes it — they do not conflict.

The surrounding erase_spaces() call is deliberately left alone, so stou32("12 34") still yields 1234. That is a separate question from partial parsing — note stou64 rejects " 100 " outright — and I did not want two behaviour changes in one commit.

The existing StringUtils.stou32 case covers a negative value, overflow and the maximum, none of which change. Adds StringUtils.stou32RejectsPartialParse for the trailing-garbage spellings plus plain numbers.

I have no OVMS build container available, so this is not compiled against the full tree and CI will need to confirm the build. I did check the change in isolation and confirmed the existing test inputs are unaffected. Draft for that reason.

🧪 Checklist

• Unit tests added.
• The documentation updated.
• Change follows security best practices.