HAPI FHIR: ReDoS via FHIRPath matches()/replaceMatches() in FHIR Validator HTTP Endpoint

Summary All implementations of FHIRPathEngine accept arbitrary FHIRPath expressions and evaluate them without input validation. The FHIRPath functions matches, matchesFull, and replaceMatches pass user-controlled regular expressions directly to Java's Pattern.compile and String.replaceAll without...