Improve test coverage for streaming messages

[lukemelia]

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[github-actions]

Host Test Results

    1 files  ±  0      1 suites  ±0   22m 47s ⏱️ +52s
743 tests +50  741 ✔️ +49  2 💤 +2  0 ❌ ±0 
748 runs  +51  746 ✔️ +51  2 💤 +2  0 ❌  - 1 

Results for commit 17e7501. ± Comparison against base commit 316d35f.

This pull request removes 1 and adds 51 tests. Note that renamed tests count towards both.

Chrome ‑ error

Chrome 132.0 ‑ Acceptance | AI Assistant tests: attaches a card in a conversation multiple times
Chrome 132.0 ‑ Acceptance | AI Assistant tests: displays active LLM in chat input
Chrome 132.0 ‑ Acceptance | Freestyle: smoke check
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests > when the user lacks write permissions: it is reflected in the realm header
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: can navigate file tree, file view mode is persisted in query parameter
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: can open files
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: can open files in base realm
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: can scroll to bottom of the file tree
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: navigating to a file in a different realm causes it to become active in the file tree
Chrome 132.0 ‑ Acceptance | code submode | file-tree tests: open directories are persisted
…