Fix : make playwright tests idempotent (#13332)

### What problem does this PR solve?

Playwright tests previously depended on cross-file execution order
(`auth -> provider -> dataset -> chat`).
This change makes setup explicit and idempotent via fixtures so tests
can run independently.

- Added/standardized prerequisite fixtures in
`test/playwright/conftest.py`:
- `ensure_auth_context`, `ensure_model_provider_configured`,
`ensure_dataset_ready`, `ensure_chat_ready`
- Made provisioning reusable/idempotent with `RUN_ID`-based resource
naming.
- Synced auth envs (`E2E_ADMIN_EMAIL`, `E2E_ADMIN_PASSWORD`) into seeded
creds.
- Fixed provider cache freshness (`auth_header`/`page` refresh on cache
hit).

Also included minimal stability fixes:
- dataset create stale-element click handling,
- search wait logic for results/empty-state,
- agent create-menu handling,
- agent run-step retry when run UI doesn’t open first click.

### Type of change

- [x] Test fix
- [x] Refactoring

---------

Co-authored-by: Liu An <asiro@qq.com>

README_ar.md

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-  <a href="./README.md"><img alt="README in English" src="https://img.shields.io/badge/English-DBEDFA"></a>
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-  <a href="./README_ar.md"><img alt="README in Arabic" src="https://img.shields.io/badge/Arabic-DFE0E5"></a>
+  <a href="./README_ar.md"><img alt="README in Arabic" src="https://img.shields.io/badge/Arabic-DBEDFA"></a>
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