TEST-202 docs: add the assistant Test tab page

[scott-lowe-vapi]

Description

Draft: merge when enable-assistant-test-tab reaches customers. It is internal-only today. Linear: TEST-202. Stacked on #1342, whose Composer section this page links to.

• New page Test an assistant (/observability/simulations-test-tab), added to the nav after Quickstart. It covers:
  • prerequisites and permissions
  • the published-version note
  • Generate with Composer
  • the suite table columns
  • when Run starts immediately and when it opens the dialog
  • Rerun
• The quickstart gets a one-line pointer to it.
• Two screenshot placeholders ({/* */}) to fill in before merge.

Labels come from apps/dashboard/src/pages/Assistants/AssistantViewer/AssistantSimulationsTab/ and features/simulationsOnboarding/constants.ts.

After merge: retarget the Test tab empty state's "About simulations" link in the dashboard (AssistantSimulationsTab/constants.ts) to this page.

What's New blurb (for the week it ships):

Assistant Test tab: Assistants now have a Test tab that lists the simulation suites assigned to them, with each suite's latest status, progress, and last run time. Select Generate with Composer to have Composer draft test requirements, generate a suite with tools that reach your systems mocked by default, and run it. Most suites start their first run immediately in chat, and Rerun lets you choose chat or voice and the number of iterations. Runs use the published version of the assistant, and each suite links to its run history in Simulations.

Testing Steps

• fern check (5.151.1) passes: 0 errors. The 14 warnings were already on main.
• Re-check every label against the dashboard on the day the flag widens
• Add the screenshots

[github-actions]

🌿 Preview your docs: https://vapi-preview-01a1526d-62a3-747b-b42f-727f64b2f8dc.docs.buildwithfern.com

[scott-lowe-vapi]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• TEST-202 docs: add the assistant Test tab page #1343 👈 (View in Graphite)
• TEST-201 docs: bring Simulations pages up to date with Composer test generation and the run API #1342 : 1 other dependent PR (#1344 )
• main

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🌿 Preview your docs: https://vapi-preview-01a1527c-adb7-7417-b24e-0fd9fb012bbe.docs.buildwithfern.com

[github-actions]

🌿 Preview your docs: https://vapi-preview-01a1527c-ebf0-776b-8a1b-ca9787650cb8.docs.buildwithfern.com