* test(e2e): cover add-fallback flow in Router Settings as proxy admin

The Router Settings → Fallbacks → Add Fallbacks flow was an uncovered
manual-QA path. This adds a test that opens the modal, picks a primary
+ fallback from the seeded mock models, saves, and verifies both render
in the fallback table.

* fix(e2e): make router-fallback test idempotent and pick antd options by text

- Match `.ant-select-item-option` by text instead of `getByTitle(...)` —
  FallbackGroupConfig uses `options=` (not <Select.Option> children), so
  no `title` attribute is emitted and the title-based selector hangs.
- Add before/after hooks that wipe any fallback for fake-openai-gpt-4 via
  /config/update so retries and local reruns don't trip on leftover state.
- Tighten the success assertion to a single tbody row containing BOTH the
  primary and the fallback names — pre-existing rows can no longer
  vacuously satisfy the check.
- Fix the stale "Three tabs" comment to "Four tabs".

Addresses Greptile P2s on PR BerriAI#29069.

* fix(e2e): keyboard-select fallback models + correct cleanup endpoint

- Replace mouse-based option clicks with click-to-focus + type + Enter.
  FallbackGroupConfig's Selects use `options=` and a custom
  getPopupContainer, so locating options via `.ant-select-dropdown`
  hit several races: DOM-clicks left antd's popup state stale (the
  primary popup then intercepted the fallback click), `getByRole`
  matched always-mounted hidden options, and pointer stability fought
  the open animation. Typing into the showSearch input narrows the
  listbox to one option and Enter selects it cleanly.
- Assert on dialog-side state changes (the active tab adopts the
  primary model name; the chain helper shows "1/10 used") instead of
  popup contents — these reflect the actual selection landing.
- Cleanup helper now hits /get/config/callbacks (the real endpoint;
  /get/callbacks returns 404), so the before/after reset actually
  clears prior router_settings.fallbacks state.