fix(desktop): preserve interim assistant text

[sweetcornna]

Fixes #39558.

Summary

• preserve text parts that streamed before the last tool call when message.complete replaces the final assistant text
• keep the existing behavior for normal no-tool completions by replacing all streamed text when no tool call was present
• add a hook-level regression test for message.delta -> tool.start -> tool.complete -> message.complete

Red

• npm run test:ui -- src/app/session/hooks/use-message-stream.test.tsx
  • failed with parts ['tool-call', 'text'] instead of ['text', 'tool-call', 'text']

Proof

• npm run test:ui -- src/app/session/hooks/use-message-stream.test.tsx src/lib/chat-messages.test.ts
• npx eslint src/app/session/hooks/use-message-stream.ts src/app/session/hooks/use-message-stream.test.tsx
• npm run type-check
• git diff --check (only Windows CRLF warning for apps/desktop/src/app/session/hooks/use-message-stream.ts)

[sweetcornna]

Added follow-up ea40adcf1 after auditing a completion-shape edge case.

The previous fix preserved text before the last tool call, but if message.complete.text contains the full final assistant text rather than only the post-tool tail, Desktop could render duplicated pre-tool text, e.g. Planning. Planning. Done.. I added a RED regression for that shape and now trim already-rendered kept text from the completion before appending the final tail.

Fresh validation on this head:

npm run test:ui -- src/app/session/hooks/use-message-stream.test.tsx
# 2 passed

npm run test:ui -- src/app/session/hooks/use-message-stream.test.tsx src/lib/chat-messages.test.ts
# 2 files passed, 26 tests passed

npx eslint src/app/session/hooks/use-message-stream.ts src/app/session/hooks/use-message-stream.test.tsx
# passed

npm run type-check
# passed

git diff --check
# passed; Windows CRLF working-copy warnings only

The PR is still mergeable on GitHub after the push.

[teknium1]

Thanks for the focused regression coverage. The reported behavior is still present on current main: apps/desktop/src/app/session/hooks/use-message-stream/index.ts:363-377 removes every streamed text part when a completion lands, while index.ts:294-328 preserves pre-tool text and tool ordering before that completion.

Problems

• The PR edits the pre-refactor path apps/desktop/src/app/session/hooks/use-message-stream.ts; current main moved the stateful hook to use-message-stream/index.ts in 086343854 and then extracted event dispatch in 51a710e57.
• The test harness is also stale: current MessageStreamOptions requires sessionStateByRuntimeIdRef at use-message-stream/index.ts:43-48.

Suggested changes

• Port the retention/deduplication logic into current index.ts, operating on its existing visibleFinalText value at line 360 so generated-image filtering remains intact.
• Update the regression test for the folder layout and current hook options; retain both completion shapes covered by the PR.

Automated hermes-sweeper review.

[teknium1]

Current main's MessageStreamOptions requires sessionStateByRuntimeIdRef (apps/desktop/src/app/session/hooks/use-message-stream/index.ts:43-48), so this harness needs that ref when the test is ported to the current hook layout.

[sweetcornna]

Closing in favor of merged #65919, which supersedes this earlier attempt and preserves my contribution in the co-author credits. Thanks for consolidating the fix across the agent, gateway, Desktop, and TUI layers.