fix(snippetgen): don't create duplicate requests for required oneofs

[busunkim96]

Some APIs mark every field in a oneof as "required" to express that the oneof itself is required.

https://github.com/googleapis/googleapis/blob/55726d62556966c095a096aed1ffda5da231f36d/google/cloud/channel/v1/service.proto#L1057-L1069

// Request message for [CloudChannelService.ImportCustomer][google.cloud.channel.v1.CloudChannelService.ImportCustomer]
message ImportCustomerRequest {
  // Specifies the identity of the transfer customer.
  // A customer's cloud_identity_id or domain is required to look up the
  // customer's Cloud Identity. For Team customers, only the cloud_identity_id
  // option is valid.
  oneof customer_identity {
    // Required. Customer domain.
    string domain = 2 [(google.api.field_behavior) = REQUIRED];

    // Required. Customer's Cloud Identity ID
    string cloud_identity_id = 3 [(google.api.field_behavior) = REQUIRED];
  }
...

This causes an error in the current logic since two requests are generated for the same field.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• tests/snippetgen/goldens/mollusca_generated_mollusca_v1_snippets_one_of_method_required_field_async.py:26, tag mollusca_generated_mollusca_v1_Snippets_OneOfMethodRequiredField_async
• tests/snippetgen/goldens/mollusca_generated_mollusca_v1_snippets_one_of_method_required_field_sync.py:26, tag mollusca_generated_mollusca_v1_Snippets_OneOfMethodRequiredField_sync

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