fix(desktop): preserve interim assistant text

[superposition]

Fixes #40903

Summary

• preserve earlier assistant text parts when a streamed turn completes after tool calls
• replace only the current assistant text segment instead of dropping every text part in the live bubble
• add regression coverage for final-segment and full-stream completion payloads

Test plan

• npm run test:ui -- src/lib/chat-messages.test.ts
• npm run type-check
• npx eslint src/lib/chat-messages.ts src/lib/chat-messages.test.ts src/app/session/hooks/use-message-stream.ts
• git diff --check

Note: full npm run lint currently reports unrelated existing lint errors outside the touched files.

[tonydwb]

Code Review Summary

Verdict: Approved

Refactors assistant text finalization into a shared helper and removes an inline dedupe branch.

What changed

• Extracted finalizeAssistantTextParts from inline logic in use-message-stream.ts into chat-messages.ts
• Tests cover: partial text before tool calls, duplication when completion text equals streamed text

Looks Good

• Shared helper is a clean DRY improvement
• Dedupe logic is now unit-testable in isolation
• No behavior change beyond cleanup — low risk

Minor Note

• The normalizeTextForDedupe whitespace collapse (/\s+/g → ' ') is intentionally aggressive. If future tool-parts include meaningful whitespace formatting, this may need revisiting. Not a blocker here.

---

Reviewed by Hermes Agent

[teknium1]

Thanks for isolating the text-part finalization behavior. The underlying Desktop defect still exists on current main: apps/desktop/src/app/session/hooks/use-message-stream/index.ts:363-377 removes every live text part and replaces them with only the terminal completion payload.

Problems

• The PR edits the pre-refactor use-message-stream.ts; current main moved this code to apps/desktop/src/app/session/hooks/use-message-stream/index.ts in 086343854, and GitHub reports this PR as conflicting.
• A port must preserve current generated-image handling at index.ts:360 (stripGeneratedImageEchoes(...)), which was added after this PR's base.
• The new tests cover the helper only, not the live message.delta → tool boundary → message.complete path dispatched at gateway-event.ts:313-335.

Suggested changes

• Port the segment-preserving merge into current index.ts while retaining generated-image echo stripping.
• Add an event-path regression test with interleaved text/tool parts and a final-segment completion payload.

Automated hermes-sweeper review.