improve webhook version mismatch detection

.changeset/three-moons-kick.md

@@ -0,0 +1,5 @@
+---
+'@team-plain/typescript-sdk': patch
+---
+
+Fix a bug with webhook version mismatch detection

src/tests/parse-webhook.test.ts

@@ -66,4 +66,22 @@ describe('Parse webhook', () => {
       'The webhook payload (version=NEW_VERSION) is incompatible with the current version of the SDK. Please upgrade both the SDK and the webhook target to the latest version. Refer to https://www.plain.com/docs/api-reference/webhooks/versions for more information. Original error: data/webhookMetadata/webhookTargetVersion must be equal to constant'
     );
   });
+
+  it('returns a version mismatch error when the webhook target version is missing', () => {
+    const invalidWebhook = {
+      ...threadCreatedPayload,
+
+      webhookMetadata: {
+        ...threadCreatedPayload.webhookMetadata,
+        webhookTargetVersion: undefined,
+      },
+    };
+
+    const result = parsePlainWebhook(invalidWebhook);
+
+    expect(result.error).instanceOf(PlainWebhookVersionMismatchError);
+    expect(result.error?.message).toBe(
+      "The webhook payload (version=unknown) is incompatible with the current version of the SDK. Please upgrade both the SDK and the webhook target to the latest version. Refer to https://www.plain.com/docs/api-reference/webhooks/versions for more information. Original error: data/webhookMetadata must have required property 'webhookTargetVersion'"
+    );
+  });
 });

src/webhooks/parse.ts

@@ -61,15 +61,18 @@ function getParseError(
   const errorMessage = getErrorMessageForHumans(payload, originalAjvError);
 
   if (isVersionMismatch(payload)) {
-    return new PlainWebhookVersionMismatchError(errorMessage, getPayloadVersion(payload) ?? '');
+    return new PlainWebhookVersionMismatchError(
+      errorMessage,
+      getPayloadVersion(payload) ?? 'unknown'
+    );
   }
 
   return new PlainWebhookPayloadError(errorMessage);
 }
 
 function isVersionMismatch(payload: Record<string, unknown>): boolean {
-  const payloadVersion = getPayloadVersion(payload);
-  return typeof payloadVersion === 'string' && payloadVersion !== getSchemaVersion();
+  const schemaVersion = getSchemaVersion();
+  return typeof schemaVersion === 'string' && schemaVersion !== getPayloadVersion(payload);
 }
 
 function getErrorMessageForHumans(