fix(desktop): preserve streamed reasoning on late fallback

[rasitakyol]

What does this PR do?

Prevents a late reasoning.available fallback from replacing reasoning that already streamed through reasoning.delta. When no reasoning delta was received, the fallback is still inserted before assistant text. The backend now sends the complete inline reasoning fallback instead of truncating it to 500 characters.

The frontend portion mechanically salvages #47183 onto the current split hook layout while preserving skyc1e's original commit authorship. The additional backend and end-to-end event coverage address #64995's truncation/remount case.

Related Issue

Fixes #64995

Salvages and extends #47183 with original author credit preserved. #64689 remains a separate MoA-reference accumulation fix.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Security fix
• Documentation update
• Tests (adding or improving test coverage)
• Refactor (no behavior change)

Changes Made

• apps/desktop/src/app/session/hooks/use-message-stream/index.ts: adds an idempotent late-reasoning fallback reducer, preserves the existing reasoning part object, and inserts a fallback before assistant text when no delta arrived.
• agent/conversation_loop.py: relays complete available reasoning without the 500-character truncation while preserving bounded subagent previews.
• Desktop and Python regressions cover streamed preservation, no-delta fallback, full payload relay, and subagent preview bounds.

How to Test

1. Stream more than 500 characters through reasoning.delta, then emit reasoning.available; verify the displayed reasoning text and part identity remain unchanged.

2. Emit assistant text followed by reasoning.available without prior reasoning deltas; verify reasoning appears before the text.

3. Run:

   scripts/run_tests.sh tests/run_agent/ tests/agent/test_reasoning_available.py tests/tools/test_delegate.py -q
   PATH=/opt/homebrew/opt/node@22/bin:$PATH npx vitest run --maxWorkers=1
   PATH=/opt/homebrew/opt/node@22/bin:$PATH npm run typecheck
   

Results:

• Python conversation/delegation coverage: 2,202 passed
• Desktop Vitest suite on supported Node 22: 1,724 passed, 1 skipped
• focused desktop event/reducer tests after final rebase: 4 passed
• desktop TypeScript typecheck passed
• ESLint, Prettier, Ruff, and git diff --check passed

A parallel full-desktop run exposed an existing cross-file isolation race in src/app/skills/index.test.tsx; the same file passed 3/3 in isolation, and the serial supported-Node run passed the complete suite.

Checklist

Code

• I've read the Contributing Guide.
• My commits follow Conventional Commits.
• I searched existing PRs; fix(desktop): show late available reasoning #47183 is explicitly salvaged with original author credit, and fix(desktop): accumulate MoA reference reasoning blocks instead of replacing #64689 covers a separate MoA path.
• My PR contains only changes related to this fix.
• The complete local Python repository suite is green; the affected Python groups are green and the known same-base environment failures are documented separately.
• I've added tests for my changes.
• Tested on macOS 26.5.2 with Python 3.11 and Node 22.22.3.

Documentation and Housekeeping

• Documentation update: N/A; no user-facing configuration changed.
• cli-config.yaml.example: N/A.
• Architecture/workflow docs: N/A.
• Cross-platform impact considered; no platform-specific I/O changed.
• Tool descriptions/schemas: N/A.

Screenshots / Logs

No UI layout changes; regression tests exercise the gateway-event timeline and reasoning-part identity directly.