Avoid relying on peek()-after-failbit semantics in ParseNumericValue

[metsw24-max]

ValueReader::ParseNumericValue previously relied on the interactionbetween std::ws, stream failbit state, and std::istream::peek() to detect trailing non-whitespace input.

On MSVC, extracting std::ws at EOF sets failbit in addition to eofbit. The previous logic still behaved correctly because peek() returns EOF when the stream is failed, but the control flow depended on subtle stream semantics.

Make the trailing-input check explicit by attempting to extract one additional non-whitespace character and distinguishing EOF from other stream failures directly.

No intended behavior change.

[Taywee]

Looks fine to me, where "fine" means "iostreams is awful, but this gets the job done".

Thanks for the PR.