… fix CI timeout (elastic#260029)

## Summary

`security_and_spaces/group8` had 262 tests taking ~40 minutes. With ~10
minutes of step overhead this consistently exceeded the 50-minute
Buildkite timeout — all tests passed but the step was killed during
post-test cleanup, triggering 3 automatic retries and wasting ~236
minutes of CI time per affected PR.

Split into two groups targeting ~20 minutes each, with minor test
quality fixes along the way.

## Changes

- **group8** — 5 `it` blocks: return rules with gaps, ordering
(asc/desc), filter by gap status, filter by aggregated status
- **group17** (new) — 5 `it` blocks: deleted rule, empty results, latest
gap timestamp, no timestamp when no gaps, invalid params
- Registered `group17` in `.buildkite/ftr_platform_stateful_configs.yml`
and `.github/CODEOWNERS`
- Replaced inline `if` auth guard with a consistent `switch` pattern
matching the rest of the file
- Split "filter by gap status" `it` block into separate `unfilled` and
`filled` assertions

(cherry picked from commit 37a00eb)

# Conflicts:
#	.buildkite/ftr_platform_stateful_configs.yml
#	.github/CODEOWNERS
#	x-pack/platform/test/alerting_api_integration/security_and_spaces/group8/tests/alerting/gap/get_rules_with_gaps.ts