docs(openapi): add 402 response to Actor execution endpoints (#2412)

Updates API schema to match changes from apify/apify-core#26917:
unauthenticated Actor execution requests now return 402 with X402
payment info instead of 401.

I have also updated the `PamentRequired` example to be more realistic.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tobice

apify-api/openapi/components/responses/PaymentRequired.yaml

@@ -1,9 +1,9 @@
-description: Payment required - the user has exceeded their usage limit or does not have enough credits.
+description: Payment required - the user has exceeded their usage limit, does not have enough credits, or the request lacks authentication and payment credentials.
 content:
   application/json:
     schema:
       $ref: ../schemas/common/ErrorResponse.yaml
     example:
       error:
-        type: actor-memory-limit-exceeded
-        message: You have exceeded your usage limit. Please consider upgrading your plan.
+        type: x402-payment-required
+        message: Please provide X402-PAYMENT-SIGNATURE header with the payment. See https://x402.org.

apify-api/openapi/components/schemas/common/ErrorType.yaml

@@ -22,3 +22,4 @@ enum:
   - unknown-build-tag
   - unsupported-content-encoding
   - user-not-found
+  - x402-payment-required

apify-api/openapi/paths/actors/acts@{actorId}@run-sync-get-dataset-items.yaml

@@ -89,6 +89,8 @@ post:
         application/json:
           schema:
             $ref: "../../components/schemas/common/errors/ActorErrors.yaml#/ActorRunFailedError"
+    "402":
+      $ref: ../../components/responses/PaymentRequired.yaml
     "404":
       $ref: ../../components/responses/NotFound.yaml
     "408":

apify-api/openapi/paths/actors/acts@{actorId}@run-sync.yaml

@@ -62,6 +62,8 @@ post:
         application/json:
           schema:
             $ref: "../../components/schemas/common/errors/ActorErrors.yaml#/ActorRunFailedError"
+    "402":
+      $ref: ../../components/responses/PaymentRequired.yaml
     "404":
       $ref: ../../components/responses/NotFound.yaml
     "408":