CVE-2026-49485: org.hl7.fhir.core: ReDoS via FHIRPath matches()/replaceMatches() in FHIR Validator HTTP Endpoint

All implementations of FHIRPathEngine accept arbitrary FHIRPath expressions and evaluate them without input validation. The utility intended to secure this evaluation did so incorrectly, and did not fully cover all places in which evaluation was being done. An attacker can send a resource containing an evil regex pattern that causes catastrophic backtracking, exhausting system resources, and causing Denial-of-Service.