chore: regenerate libraries with gapic-generator @ faa0f81

[parthea]

Commit 510faa2 contains manual changes to the post-processing files.

Commit cc4e7f0 is the result after running client library generation using gapic-generator at commit faa0f81.

The following commands were used to re-generate the libraries

pip install <path to gapic-generator @ commit faa0f81>
V=$(go run github.com/googleapis/librarian/cmd/librarian@latest config get version)
go run github.com/googleapis/librarian/cmd/librarian@${V} install
go run github.com/googleapis/librarian/cmd/librarian@${V} generate -v --all

Fixes #18163

[gemini-code-assist]

Code Review

This pull request updates the minimum required version of google-api-core to 2.28.0 across several post-processing configurations and constraints. It introduces a new compatibility module _compat.py in the google-ads-admanager package to provide fallback implementations of google-api-core features for backward compatibility. Consequently, the REST transport implementations for all Ad Manager services are refactored to utilize the centralized transcode_request helper, eliminating a significant amount of generated boilerplate code. Additionally, the gapic-generator tool is pinned to a specific git commit in librarian.yaml. There are no review comments provided, and I have no additional feedback on these changes.