sidekick: method signature parsing fails for some APIs

[noahdietz]

ibrarian: generate library "google-cloud-billing-v1" (rust): cannot find field  in method signature for method .google.cloud.billing.v1.CloudCatalog.ListServices

caused by:

https://github.com/googleapis/googleapis/blob/7af3f2c7b8927cffb548fd2cf09b04c6371437ee/google/cloud/billing/v1/cloud_catalog.proto#L48

Originally posted by @coryan in #6455

[noahdietz]

Bumped into this after merging #6451 towards #5930

[noahdietz]

@coryan feel free to close this as a dupe of 5930 if you just want to have another go instead of tracking it separately