fix(minifier): preserve Unicode escape sequences in string literals
Fixes issue where Unicode escape sequences like \uD83D\uDE42 were being
converted to actual emoji characters during minification, breaking code
that depends on the literal escape sequences.
The fix simplifies the Unicode handling in string literals to always
preserve escape sequences by outputting \ for any \u sequence.
Fixes #10353
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Co-Authored-By: Claude <noreply@anthropic.com>