fix(ai-mcp): address PR #208 review for inline Test button (#170)

- CHANGELOG: add an [Unreleased]/Added bullet for the per-row "Test" button
  (idle Test -> OK · N / Failed, tooltip, isolated per-row state).
- ai-mcp-server-row: show a red "Failed" when the request itself rejects
  (401/403/500/network), reading testMutation.isError — previously only a
  server-reported {ok:false} was surfaced and a real reject silently reverted
  to "Test". Tooltip uses error.response?.data?.message or the i18n fallback.
- ai-mcp-server-row: clarify the reset-effect comment (hasHeaders is a
  presence flag, so value-only token rotation is intentionally not reset).
- ai-mcp-server-row: drop the redundant disabled={isPending} (Mantine already
  disables a loading button).
- ru-RU: add the "No tools available" translation.
- tests: cover the request-reject failure, the empty tool list (OK · 0), and
  the reset-on-change effect (url / transport / hasHeaders) via rerender.

Note: kept the `"error" in result` guard instead of the suggested bare
`else if (result)` — optional chaining on `result?.ok` doesn't narrow the
discriminated union in the else branch, so the bare form fails tsc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/public/locales/ru-RU/translation.json

@@ -1172,5 +1172,6 @@
   "OpenAI (official)": "OpenAI (официальный)",
   "Test": "Тест",
   "Failed": "Ошибка",
-  "OK · {{count}}": "OK · {{count}}"
+  "OK · {{count}}": "OK · {{count}}",
+  "No tools available": "Нет доступных инструментов"
 }