fix(desktop): preserve streamed text when provider sends empty completion

[liuhao1024]

What does this PR do?

Preserves streamed assistant text when a provider sends a message.complete event with empty text/rendered fields. This fixes a bug where responses from NVIDIA NIM (and any provider that sends finish_reason: "stop" with no content in the final chunk) would flash briefly and then disappear from the Desktop UI.

Related Issue

Fixes #41898

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• apps/desktop/src/app/session/hooks/use-message-stream.ts:
  • replaceTextPart: early-return existing parts when finalText is empty, preserving streamed text accumulated via message.delta events
  • shouldHydrate: skip stored-session hydration when streamed text already exists but finalText is empty, preventing stale snapshot from overwriting visible response

How to Test

1. Configure a provider that sends empty content in the final message.complete chunk (e.g. NVIDIA NIM with nvidia/nemotron-3-ultra-550b-a55b)
2. Launch Hermes Desktop App
3. Send any prompt
4. Verify the assistant response remains visible after streaming completes (no flash-and-disappear)
5. Test with a normal provider (e.g. DeepSeek) to verify no regression — responses should still appear and stay
6. Test interruption: start a response, cancel mid-stream, verify partial text is preserved

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: use-message-stream.ts → replaceTextPart, completeAssistantMessage, shouldHydrate
• Callers: message.complete event handler (line 803), streaming delta handlers
• Blast radius: LOW — scoped to Desktop app's message completion path
• Related patterns: hydrateFromStoredSession fallback (line 561), completeMessage (line 491)

[teknium1]

Thanks for isolating the empty-completion path. The defect remains on current main: gateway-event.ts:334-335 passes an empty completion string through, while index.ts:363-377 removes all streamed text parts and adds none when the final text is empty. index.ts:432-457 then enables the hydration fallback for that same empty completion.

Problems

• The PR diff changes no tests. Current hook tests under apps/desktop/src/app/session/hooks/use-message-stream/ cover todo cleanup and utilities, not an empty message.complete after streamed deltas.

Suggested changes

• Salvage the preservation guard into apps/desktop/src/app/session/hooks/use-message-stream/index.ts, which replaced the old target in behavior-preserving refactor 086343854dbf723acfc529d2580c260cc3713d0b.
• Add a regression test for message.delta text followed by an empty message.complete, including the hydration outcome.

Automated hermes-sweeper review.

[teknium1]

Please add a Desktop regression test for streamed message.delta text followed by an empty message.complete. This branch is the fix's essential behavior, and the PR currently changes no test files.