Release Notes:

- Added button tu prevent idle sleep during AI response streaming


**Context / Motivation:**
When an AI agent takes a long time to generate a response, the operating
system might enter an idle sleep (or display sleep) state if there is no
user interaction (mouse or keyboard). This can interrupt the workflow.
This PR aims to prevent the system from sleeping during an active AI
turn by invoking native OS APIs, and provides a UI toggle for users to
control this behavior.

**Key Changes:**

1. **Cross-Platform Idle Sleep Prevention in GPUI (`gpui`, `gpui_*`)**
- Added a new `prevent_idle_sleep` method to the `Platform` trait. It
returns a RAII-based `PreventIdleSleepToken`, which automatically
restores the original sleep policy when dropped.
- **macOS**: Utilizes `NSProcessInfo`'s `beginActivityWithOptions` with
`NSActivityIdleDisplaySleepDisabled`.
- **Windows**: Uses `SetThreadExecutionState` to prevent display and
system sleep, incorporating a reference counter to handle concurrent
requests safely.
- **Linux**: Uses `ashpd` to request `org.freedesktop.portal.Inhibit`
(inhibiting `Idle` and `Suspend` states) via D-Bus.

2. **AI Agent State Integration (`crates/acp_thread`)**
- Introduced idle sleep prevention logic in `AcpThread`: Acquires the
token at the beginning of `run_turn`, and automatically releases it when
the turn completes normally, errors out, or is explicitly canceled by
the user.
- Added comprehensive unit tests to ensure the token is correctly
acquired and released under various cancellation and concurrency
scenarios.

3. **Settings and UI (`crates/agent_settings`, `crates/agent_ui`)**
   - Added `AgentOutputIdleSleepControl` to global state.
- Added a new toggle button (screen icon) to the toolbar at the bottom
of the `ThreadView` (AI conversation panel). Users can hover to view the
tooltip and click to toggle the feature on or off.

<img width="419" height="132" alt="截屏2026-04-04 16 09 08"
src="https://github.com/user-attachments/assets/619da9ff-10f7-46e0-b959-d7c74b16b918"
/>

---------

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>