ci: editing a disabled chainsaw suite escalates test selection to the full e2e run #3655

Summary

Editing hack/e2e-chainsaw/backup/chainsaw-test.yaml.disabled escalates test selection to the full e2e run. The selector's suite-mapping step derives the suite name from the path without regard to the .disabled suffix, so the change selects the backup suite; the final intersection against suites that actually exist comes up empty, and the empty-selection safety net fires, which means the full run. Measured on the fail-safe selection branch currently in flight.

Why it matters

A disabled suite is the one file class whose edits provably cannot affect any running test, and it currently buys the most expensive outcome the selector has. The practical annoyance is small (edits to disabled suites are rare), but the mapping is exactly backwards, and anyone re-enabling or grooming the disabled suite pays a full run per commit while doing it.

Fix shape

Teach the suite-mapping step to recognize *.disabled and classify such paths as inert with a comment stating why (a disabled suite runs nowhere, so nothing can regress), or map it to the suite it would become and let the intersection drop it silently without triggering the safety net. Either way the safety net keeps its job for paths that are genuinely unclassified.