[Flaky Test Fixer] Open a draft fix PR for issues labeled ai:auto-flaky-fix (elastic#268810)

This PR adds a new agentic workflow that fires when a `failed-test`
issue is labeled `ai:auto-flaky-fix`.

- It reads the latest Failed Test Investigator comment to pick up the
suspected root cause
- It applies the smallest test-side patch it can justify
- It verifies the fix locally and opens a draft PR labeled
`auto:flaky-fix` if successful.

> [!NOTE]
> As of today, we're expecting the `ai:auto-flaky-fix` label to me added
**manually** - as a next step, the failed test investigator workflow
will add the label once we're confident flaky test fix PRs quality is
good enough.

Author: csr