test(ci): strict nextest ci profile with quarantine + flake policy (#4666)

test(ci): add strict nextest ci profile with quarantine + flake policy

Formalize the existing ecstore-serial-flaky mechanism into a strict CI gate
(ci-10, absorbs infra-15; backlog#1149).

- .config/nextest.toml: add [profile.ci] with global retries=0 (never mask a
  new race's first occurrence), fail-fast=false, and JUnit output at
  target/nextest/ci/junit.xml. Add a quarantine section where flaky tests get
  retries=2 under the ci profile only; each entry links one OPEN issue. First
  members are the two backlog#937 ecstore groups
  (concurrent_resend_same_part_commits_one_generation and
  store::bucket::tests::bucket_delete_*), which keep their existing
  ecstore-serial-flaky test-group serialization. Local default profile still
  never retries.
- .github/workflows/ci.yml: run the main test step with --profile ci and upload
  the JUnit report (if: always(), 3-day retention, run-number in name). The
  migration-proof step stays on the default profile to avoid clobbering the ci
  JUnit artifact (its tests are not quarantined).
- docs/testing/README.md: new skeleton (owned by backlog#1153 infra-11) holding
  the flake policy: discover -> open issue within 24h -> quarantine with issue
  link -> fix or delete within 30 days. AGENTS.md points to it.

Refs: rustfs/backlog#1149, rustfs/backlog#937, rustfs/backlog#1155

.gitignore

@@ -55,6 +55,8 @@ docs/*
 !docs/architecture/**
 !docs/operations/
 !docs/operations/**
+!docs/testing/
+!docs/testing/**
 !docs/superpowers/
 !docs/superpowers/**
 docs/heal-scanner-logging-governance.md