Fix unhandled NumberFormatException in webhook timestamp parsing

When parsing the Stripe-Signature header, getTimestamp() calls Long.parseLong()
on the timestamp value without catching NumberFormatException. If a malformed
header contains a non-numeric timestamp value (e.g., 't=not_a_number'), the
exception propagates up uncaught, rather than being wrapped in a
SignatureVerificationException as documented.

This commit catches NumberFormatException and returns -1, which causes
verifyHeader() to throw the expected SignatureVerificationException with
the message 'Unable to extract timestamp and signatures from header'.

Also adds a test case for malformed timestamp values.

Author: Nepomuk5665

src/main/java/com/stripe/net/Webhook.java

@@ -176,7 +176,11 @@ private static long getTimestamp(String sigHeader) {
       for (String item : items) {
         String[] itemParts = item.split("=", 2);
         if (itemParts[0].equals("t")) {
-          return Long.parseLong(itemParts[1]);
+          try {
+            return Long.parseLong(itemParts[1]);
+          } catch (NumberFormatException e) {
+            return -1;
+          }
         }
       }
 

src/test/java/com/stripe/net/WebhookTest.java

@@ -160,6 +160,21 @@ public void testMalformedHeader() throws SignatureVerificationException {
     assertEquals("Unable to extract timestamp and signatures from header", exception.getMessage());
   }
 
+  @Test
+  public void testMalformedTimestampValue() throws SignatureVerificationException {
+    // Test with non-numeric timestamp value - should throw SignatureVerificationException,
+    // not NumberFormatException
+    final String sigHeader = "t=not_a_number,v1=somesignature";
+
+    Throwable exception =
+        assertThrows(
+            SignatureVerificationException.class,
+            () -> {
+              Webhook.Signature.verifyHeader(payload, sigHeader, secret, 0, null);
+            });
+    assertEquals("Unable to extract timestamp and signatures from header", exception.getMessage());
+  }
+
   @Test
   public void testNoSignaturesWithExpectedScheme()
       throws SignatureVerificationException, NoSuchAlgorithmException, InvalidKeyException {