Avoid redundant whitespace scan in json.loads() for documents without surrounding whitespace

Decoding a JSON document with json.loads() runs a whitespace-skipping regular expression at the start and end of the document through the pure-Python decode() wrapper, on every call, before and after the C scanner does the actual parsing. The overwhelming majority of documents have no leading or trailing whitespace, so both matches scan zero characters yet still pay for the call and the match-object allocation. For the small documents that dominate real traffic, that fixed overhead is a meaningful fraction of the total decode time.

Skipping the leading match when the document does not begin with whitespace, and the trailing match when the parse already consumed the whole string, removes that overhead from the common case. On a tiny document it is roughly 1.5x faster; documents that do have surrounding whitespace keep the original behavior, and large documents are unaffected.

This overlaps with the broader rewrite proposed in gh-117397; it is filed separately so the small, self-contained version can be evaluated on its own.

Linked PRs

• gh-150861

json.loads() とその pure-Python の decode() ラッパーから始め、次に issue で説明されている C スキャナーおよび gh-117397 のより広範な書き換えと、空白の処理を比較します。周囲に空白がないドキュメントでは冗長なスキャンを回避し、空白があるドキュメントではこれまでの動作を維持できれば完了です。gh-150861 は、すでに作業が進行中であることを示しています。