fix(app): expose streaming assistant content - #48597

Issue for this PR

Fixes #33137
Fixes #46396
Addresses the streaming-content half of #41408; its animated-number issue remains separate.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Active assistant output was wrapped in aria-hidden, so visible streamed text, reasoning, and tool activity disappeared from the accessibility tree until the turn ended.

This removes that attribute from the current app timeline and adds a Playwright ARIA-snapshot regression test for an incomplete assistant response. It deliberately leaves aria-live="off" unchanged so browsing the stream does not also announce every token.

Prior work: @ashu-choudhury identified the root cause and proposed removing the attribute in #33139. @abdullahsaad27 traced the active Desktop renderer and explored the aria-busy/live-region alternative in #38393. This PR applies the narrower fix to current dev and adds regression coverage.

How did you verify your code works?

• Confirmed the new test failed before the fix with an empty ARIA snapshot, then passed after it.
• bun turbo typecheck (30 packages via the pre-push hook)
• App unit tests: 724 passed
• App browser tests: 41 passed
• Timeline accessibility E2E: 2 passed in Chromium
• Production timeline benchmark passed before and after with all 40 deltas delivered and no row or Markdown remounts

Screenshots / recordings

N/A — this changes only the accessibility tree. The regression snapshot changes from empty to paragraph: Streaming assistant output; visual rendering is unchanged.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR