docs(phonenumberverification): add descriptive javadoc to package-private constructor overload

Author: lahirumaramba

src/main/java/com/google/firebase/phonenumberverification/internal/FirebasePhoneNumberVerificationTokenVerifier.java

@@ -62,6 +62,10 @@ public FirebasePhoneNumberVerificationTokenVerifier(FirebaseApp app) {
     this.projectId = getProjectId(app);
   }
 
+  /**
+   * Package-private constructor designed explicitly for dependency injection
+   * during isolated unit testing flows.
+   */
   FirebasePhoneNumberVerificationTokenVerifier(
       String projectId, DefaultJWTProcessor<SecurityContext> jwtProcessor) {
     this.projectId = projectId;