fix: skip stale auto optimization tasks

[Pigbibi]

Summary

• exclude experiment-only tasks from auto draft PR generation
• skip known low-risk tasks that are already implemented on the current main branch
• cover the new filtering behavior in the auto optimization payload tests

Testing

• python3 -m unittest tests/test_prepare_auto_optimization_pr.py tests/test_auto_optimization_pr_workflow_config.py
• uvx ruff check scripts tests
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f) }; puts "yaml ok"'