chore: resolve comments

Author: boikoa-gl

src/main/java/com/google/firebase/fpnv/FirebasePnvException.java

@@ -24,6 +24,23 @@
 public class FirebasePnvException extends Exception {
   private final FirebasePnvErrorCode errorCode;
 
+  /**
+   * Exception that created from {@link FirebasePnvErrorCode},
+   * {@link String} message and {@link Throwable} cause.
+   *
+   * @param errorCode {@link FirebasePnvErrorCode}
+   * @param message {@link String}
+   * @param cause {@link Throwable}
+   */
+  public FirebasePnvException(
+      FirebasePnvErrorCode errorCode,
+      String message,
+      Throwable cause
+  ) {
+    super(message, cause);
+    this.errorCode = errorCode;
+  }
+
   /**
    * Exception that created from {@link FirebasePnvErrorCode} and {@link String} message.
    *
@@ -34,8 +51,7 @@ public FirebasePnvException(
       FirebasePnvErrorCode errorCode,
       String message
   ) {
-    super(message);
-    this.errorCode = errorCode;
+    this(errorCode, message, null);
   }
 
   public FirebasePnvErrorCode getFpnvErrorCode() {

src/main/java/com/google/firebase/fpnv/FirebasePnvToken.java

@@ -30,7 +30,6 @@ public class FirebasePnvToken {
   private final Map<String, Object> claims;
 
   public FirebasePnvToken(Map<String, Object> claims) {
-    // this.claims = claims != null ? Collections.unmodifiableMap(claims) : Collections.emptyMap();
     checkArgument(claims != null && claims.containsKey("sub"),
         "Claims map must at least contain sub");
     this.claims = ImmutableMap.copyOf(claims);
@@ -58,8 +57,14 @@ public List<String> getAudience() {
     Object audience = claims.get("aud");
     if (audience instanceof String) {
       return ImmutableList.of((String) audience);
+    } else if (audience instanceof List) {
+      // The nimbus-jose-jwt library should provide a List<String>, but we copy it
+      // to an immutable list for safety and to prevent modification.
+      @SuppressWarnings("unchecked")
+      List<String> audienceList = (List<String>) audience;
+      return ImmutableList.copyOf(audienceList);
     }
-    return (List<String>) audience;
+    return ImmutableList.of();
   }
 
   /**

src/main/java/com/google/firebase/fpnv/internal/FirebasePnvTokenVerifier.java

@@ -95,14 +95,20 @@ public FirebasePnvToken verifyToken(String token) throws FirebasePnvException {
     } catch (ParseException e) {
       throw new FirebasePnvException(
           FirebasePnvErrorCode.INVALID_TOKEN,
-          "Failed to parse JWT token: " + e.getMessage()
+          "Failed to parse JWT token: " + e.getMessage(),
+          e
       );
     } catch (ExpiredJWTException e) {
-      throw new FirebasePnvException(FirebasePnvErrorCode.TOKEN_EXPIRED, "FPNV token has expired.");
+      throw new FirebasePnvException(
+          FirebasePnvErrorCode.TOKEN_EXPIRED,
+          "FPNV token has expired.",
+          e
+      );
     } catch (BadJOSEException | JOSEException e) {
       throw new FirebasePnvException(FirebasePnvErrorCode.SERVICE_ERROR,
           "Check your project: " + projectId + ". "
-              + e.getMessage()
+              + e.getMessage(),
+          e
       );
     }
   }
@@ -140,7 +146,7 @@ private void verifyClaims(JWTClaimsSet claims) throws FirebasePnvException {
 
     if (Strings.isNullOrEmpty(issuer)) {
       throw new FirebasePnvException(FirebasePnvErrorCode.INVALID_ARGUMENT,
-          "Invalid issuer. Expected: " + issuer);
+          "FPNV token has no 'iss' (issuer) claim.");
     }
 
     // Verify Audience