[rust-compiler] Bridge: sanitize lone surrogates in JSON, with escaped-backslash protection

Summary:
Combines initial sanitization (replacing lone surrogates with �) and
regex lookbehind protection (to avoid matching escaped-backslash sequences
like literal "\uD83D" in source's extra.raw fields).

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Differential Revision: https://phabricator.intern.facebook.com/D102193138