Document the "Failed data" tab (failure store) in the Edit data lifecycle flyout

## Summary

[`manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md`](https://github.com/elastic/docs-content/blob/main/manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md) documents the **Successful data** tab of the **Edit data lifecycle** flyout (Index Management → Data Streams → a data stream's details → **Actions** → **Edit data lifecycle**), but has no coverage at all of the flyout's second tab, **Failed data** (the failure store). This isn't a stale-content problem like the Frozen phase one (#7953) — the page has zero mentions of failure store handling via this UI.

## What's in the Failed data tab (verified in source)

- Its own **Inherit lifecycle from index template** checkbox, independent of the Successful data tab's (confirmed: `inheritSuccessfulLifecycle`/`inheritFailedLifecycle` are separate state in `use_edit_data_lifecycle.ts`).
- A separate **Enable failure store** checkbox (disabled while Inherit is on), with help text: *"Disabling this feature will not delete existing data within your failure store."*
- When failure store is enabled, a **Data phases** panel appears with only a **Delete phase** card (no Hot/Frozen — failure store only ever has retention, no tiering).
- **Serverless-specific behavior**: when failure store is enabled, the Delete phase's toggle is hidden and force-enabled (`hideToggle`/`forceFailedDeletePhaseEnabled` in `edit_data_lifecycle_flyout.tsx`) — a retention period is mandatory, you cannot keep failed data indefinitely, unlike Stack.

## Source pointers (Kibana, `elastic/kibana`)

- `x-pack/platform/packages/shared/kbn-data-lifecycle-phases/src/edit_failed_data_lifecycle_flyout/edit_failed_data_lifecycle_flyout_body.tsx` and its `strings.ts`.
- `x-pack/platform/plugins/shared/index_management/public/application/components/data_lifecycle/edit_data_lifecycle_flyout/edit_data_lifecycle_flyout.tsx` — the `forceFailedDeletePhaseEnabled` / `hideToggle` serverless gating.
- `x-pack/platform/plugins/shared/index_management/public/application/sections/home/data_stream_list/data_stream_detail_panel/lifecycle/use_edit_data_lifecycle.ts` — `flyoutFailedData` memo, `inheritFailedLifecycle`, `handleInheritFailedLifecycleChange`.

## Where this was found

Found while verifying [PR #7901](https://github.com/elastic/docs-content/pull/7901) (issue #7637), which documents the Successful data tab's ILM policy method and its Managed-policy filter. Not fixed there since failure store never uses ILM at all — this is a fully separate, unrelated piece of UI.

## Suggested scope

A new subsection (or standalone section) on `tutorial-update-existing-data-stream.md` covering: opening the **Failed data** tab, the Inherit checkbox and its independence from Successful data, enabling the failure store, setting a retention period via the Delete phase, and the Stack-vs-Serverless difference on mandatory retention. Needs its own `applies_to` floor verification (when this UI shipped) before tagging — don't assume it matches the 9.5.0 Successful-data redesign without checking.

Update manage-data/lifecycle/data-stream/tutorial-update-existing-data-stream.md, using its Successful data coverage as the starting point. Read the linked Kibana source files to verify the Failed data behavior and determine the correct applies_to floor before documenting the tab, inheritance, failure-store enablement, Delete phase retention, and Stack-versus-Serverless behavior. Done means the page covers these items accurately and distinguishes failure-store handling from ILM.

Written by the indexing model from the issue text.