chore: migrate google-ads-admanager to librarian

[jskeet]

No description provided.

[gemini-code-assist]

Code Review

This pull request enables client library generation for Google Ad Manager by removing the skip_generate flag and cleaning up various boilerplate and configuration files. Additionally, it updates the repository metadata, introduces the AdManagerError message definition to the protobuf configuration, and integrates the new admanager_error.proto across all service proto lists. I have no further feedback to provide as there are no review comments to address.

[jskeet]

I think this is okay - admanager_error.proto isn't listed in https://github.com/googleapis/googleapis/blob/master/google/ads/admanager/v1/BUILD.bazel, but is present in the directory... it's reasonable for us to generate all the proto files.

[JoeWang1127]

Is this a new package?

[jskeet]

Is this a new package?

No, it's a single new proto that isn't in the Bazel list of protos - we find all the protos in the API directory, hence the difference.