feat(ci): per-config failing-test names in FTR job annotation

[TamerlanG]

Summary

Adds structured per-config test-failure details to the Buildkite FTR job annotation, without any shell temp-file plumbing.

How it works:

• New ftr_annotation_helper.ts exports two functions:
  • snapshotJunitDir(dir) — records current *.xml paths to <dir>/.ftr_annotation_snapshot
  • collectNewFailedTestNames(dir) — diffs snapshot vs current XMLs, parses failures from new files only, deletes snapshot
• scripts/ftr_annotation_helper.js wires up snapshot <dir> and list-new-failures <dir> CLI subcommands
• ftr_configs.sh calls snapshot before each config run and list-new-failures in the failure branch to get per-config failing test names
• ftr_job_annotation.sh contains write_job_annotation() which emits a Buildkite annotation with:
  • Per-config status table (passed / recovered / failed / still-failing / new-failure)
  • Per-config bulleted list of failing test names

Annotation example:

### FTR Configs — `my-group` (attempt 1)

| Config | Duration | Status |
| --- | --- | --- |
| `x-pack/test/foo/config.ts` | 4m 12s | **failed** |
| `x-pack/test/bar/config.ts` | 1m 3s | passed |

**Failing tests — `x-pack/test/foo/config.ts`:**

- Suite › my test name

Test plan

• Unit tests: node scripts/jest packages/kbn-failed-test-reporter-cli/failed_tests_reporter/ftr_annotation_helper.test.ts (5 tests, all green)
• CI: push to branch and verify annotation appears in a real FTR step with failing configs

Related

Companion to #269605 (smart retry). This PR adds the annotation layer on top of main independently, so it can land separately.

[infra-vault-gh-plugin-prod]

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