Edit tool's \uXXXX-escape text search hardened with a length guard and clearer failure

The Edit tool can treat a literal search string containing \uXXXX unicode escape sequences as if it were a regular expression. Before building that regular expression, Claude Code now estimates how long the resulting pattern would be and checks that the escape sequences are well-formed, instead of just testing length and regex validity generically. If this check fails, it now logs a specific message explaining that the \uXXXX-escape search was skipped because the regex engine refused a pattern of a given length, rather than silently returning nothing.