CI cleanup: split repo-level test workflows and narrow plugin fan-out

[lucarlig]

Summary

The current Rust/Python plugin CI mixes plugin-scoped validation with repo-level test coverage, which causes unnecessary matrix fan-out and makes workflow intent harder to follow.

This cleanup would separate repo-level test suites into their own workflows and keep plugin CI focused on plugin-specific changes.

Current behavior

The main plugin CI workflow currently handles both:

• plugin selection and per-plugin build/test execution
• repo-level test files such as:
  • tests/test_plugin_catalog.py
  • tests/test_install_built_wheel.py

This makes the workflow broader than it needs to be and contributes to CI running more work than necessary for plugin-focused PRs.

Desired behavior

• Main plugin CI should only cover plugin-scoped validation
• Repo-level test suites should run in dedicated workflows
• Plugin integration tests under plugins/tests/<slug>/... should remain plugin-scoped
• Only truly shared repo inputs should trigger all-plugin fan-out

Proposed changes

• Narrow shared-path detection in tools/plugin_catalog.py
• Stop treating Cargo.lock as a shared all-plugin trigger
• Stop treating root tests/** as shared plugin-CI input
• Map plugins/tests/<slug>/... to the owning plugin
• Create a dedicated workflow for tests/test_plugin_catalog.py
• Create a dedicated workflow for tests/test_install_built_wheel.py
• Keep the main plugin CI workflow focused on:
  • plugin source changes
  • plugin integration tests
  • true shared repo inputs such as shared crates, workflows, and tooling

Expected outcome

• Cleaner CI workflow boundaries
• Less unnecessary plugin matrix execution
• Clear separation between plugin CI and repo-level validation
• Easier maintenance of workflow path contracts and plugin selection logic

Acceptance criteria

• A change in plugins/rust/python-package/<slug>/... runs CI only for <slug>
• A change in plugins/tests/<slug>/... runs CI only for <slug>
• Cargo.lock does not cause all-plugin fan-out by itself
• tests/test_plugin_catalog.py runs in its own workflow
• tests/test_install_built_wheel.py runs in its own workflow
• Main plugin CI no longer directly runs those repo-level test files
• Workflow contract tests cover the new routing behavior