@grpc/grpc-js: safe_regex incorrectly applies partial string matching for patterns with alternation

Summary The StringMatcher/PathMatcher for saferegex builds its RE2 as ^$regex$ with no surrounding group. Because alternation has the lowest precedence, a pattern "a|b|c" compiles to /^a|b|c$/ -- "starts with a" OR "contains b" OR "ends with c" instead of a full match of the whole value...