fix(chat): preserve reasoning and simplify streaming indicators

[mrsibe]

What does this PR do?

Keeps the reasoning component visible after an assistant reply finishes, defaults it to collapsed, and removes the redundant thinking placeholder and rectangular streaming cursors.

Why?

The renderer removed the live turn after applying its outcome but only copied the answer text into the settled message. Reasoning was consequently lost from the current transcript until messages were reloaded. Automatic expansion and duplicate streaming indicators also made the chat unnecessarily noisy.

What changed?

• Preserve assembled reasoning alongside answer text when settling a turn.

• Show the reasoning component while reasoning is live, even before its first text arrives.

• Keep reasoning collapsed by default and respect the user's expansion choice across streaming state changes.

• Remove the separate “正在思考” placeholder and rectangular cursors from answer and reasoning content.

• Add a regression test delivering the outcome immediately after the chunks to verify that final reasoning survives assembler draining.

• Fix Windows parser tests loading Electron and triggering a binary download: load the Electron app only inside an Electron process, with a regression test.

• Fix the macOS smoke-test race by waiting for watched-folder background imports to finish before deleting their source.

How was this tested?

• npm run typecheck — passed.
• npm test — all 486 tests passed.
• npm run build — passed.
• npm run build:unpack — passed on Linux.
• npm run smoke:packaged — all 29 packaged-app checks passed on Linux.
• Targeted ESLint and Prettier checks for the changed files — passed.
• git diff --check — passed.
• Desktop interaction and visual verification have not been performed.

Screenshots / recordings

Not captured. Manual desktop verification remains outstanding: confirm reasoning stays visible after completion, starts collapsed, can be expanded manually, and no rectangular cursor or separate thinking placeholder is displayed.

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary (no documentation changes needed).

Desktop / build changes

• npm run build:unpack passes (Linux).
• npm run smoke:packaged passes (Linux).
• Windows and macOS validation is pending the new CI run.