feat: return SamlMessage from binding receive() method

[william-suppo]

Summary

• AbstractBinding::receive() now returns SamlMessage instead of void
• Both HttpPostBinding and HttpRedirectBinding return the deserialized message

Context

The official documentation example showed:

$response = $binding->receive($request, $messageContext);
print $response->getID(); // Fatal error: $response is null

Since receive() returned void, $response was always null. Users had to know to call $messageContext->getMessage() instead — which is not documented.

With this change, both patterns work:

// New: assign directly from receive()
$message = $binding->receive($request, $messageContext);

// Existing: still works unchanged
$binding->receive($request, $messageContext);
$message = $messageContext->getMessage();

This is non-breaking: existing code that ignores the return value continues to work.

Closes #89