Fix YAML send: with associative arrays

Bug Fixes

• Fixed TypeError in resolveInitialPayload() - The method now accepts both string and array types, allowing YAML flow definitions to use send: with associative arrays directly.

Details

Previously, when using a YAML flow definition with a send: block containing key-value pairs:

send:
  name: "John"
  email: "john@example.com"

The flow would throw a TypeError: Argument #1 ($payloadClass) must be of type string, array given because the method only accepted strings.

This release fixes the issue by updating the method signature to string|array and handling arrays as direct payload data.

Fixes #55