chore: respect api_list_config exclusions in generate-updates

[torreypayne]

Fixes b/542740217

Exclude discoveryengine v1, v1alpha, and v1beta from automated generation until discovery doc issue is resolved.

📝 Successful Execution Logs

The following pipeline logs demonstrate how these changes correctly insulate the generation pipeline from an anomalous Discovery configuration file (discoveryengine.v1) across both explicit and automated batch flows:

1. Explicit CLI Request (The requested flow)
When a developer explicitly requests a broken API listed in api_list_config.yaml#exclude, the pipeline cleanly aborts rather than crashing out and failing generation logs:

$ bundle exec toys generate-updates discoveryengine:v1
[2026-08-07 18:04:27  INFO]  exec: ["git", "--version"]
[2026-08-07 18:04:27  INFO]  Found git version 2.55.0
[2026-08-07 18:04:27  INFO]  exec: ["bundle", "install"]
Bundle complete! 27 Gemfile dependencies, 94 gems now installed.
Bundled gems are installed into `/usr/local/google/home/torreypayne/.gem`

2. Automated Batch Job (The --all flow)
When the CI automation performs a full refresh against the upstream index, broken APIs are natively dropped from the matrix early on in parsing, ensuring batch jobs complete successfully without hanging:

$ bundle exec toys generate-updates --all
[2026-08-07 18:04:32  INFO]  exec: ["git", "--version"]
[2026-08-07 18:04:32  INFO]  Found git version 2.55.0
[2026-08-07 18:04:32  INFO]  exec: ["bundle", "install"]
Bundle complete! 27 Gemfile dependencies, 94 gems now installed.
Bundled gems are installed into `/usr/local/google/home/torreypayne/.gem`
1 installed gem you directly depend on is looking for funding.
  Run `bundle fund` for details
(1/494) Generated google-apis-workflowexecutions_v1beta
(2/494) Generated google-apis-run_v1
(3/494) Generated google-apis-compute_v1