fix device_utils (#16)

This release improves async support with a new `arun()` helper, makes the Device Utilities module fully non-blocking, adds VT100 terminal emulation for screen-based commands, and introduces interactive SSH shell access for EX/SRX devices. (PR #16)

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### 1. NEW FEATURES

#### **`mistapi.arun()` — Async Helper**
New helper function to run any sync mistapi function without blocking the event loop. Wraps the function call in `asyncio.to_thread()` so blocking HTTP requests run in a thread pool.

```python
import asyncio
import mistapi
from mistapi.api.v1.sites import devices

async def main():
    session = mistapi.APISession(env_file="~/.mist_env")
    session.login()

    # Run sync API call without blocking the event loop
    response = await mistapi.arun(devices.listSiteDevices, session, site_id)
    print(response.data)

asyncio.run(main())
```

#### **Interactive SSH Shell** (`device_utils.ex` / `device_utils.srx`)
New `interactiveShell()` and `createShellSession()` functions for SSH-over-WebSocket access to EX and SRX devices.

- `interactiveShell()` — takes over the terminal for human SSH access (uses `sshkeyboard`)
- `createShellSession()` — returns a `ShellSession` object for programmatic send/recv
- `ShellSession` — bidirectional WebSocket session with `send()`, `recv()`, `resize()`, context manager support

```python
from mistapi.device_utils import ex

# Interactive (human at the keyboard)
ex.interactiveShell(apisession, site_id, device_id)

# Programmatic
with ex.createShellSession(apisession, site_id, device_id) as session:
    session.send_text("show version\r\n")
    import time; time.sleep(3)
    while (data := session.recv(timeout=0.5)):
        print(data.decode("utf-8", errors="replace"), end="")
```

#### **`topCommand`** (`device_utils.ex` / `device_utils.srx`)
New `topCommand()` function to stream `top` output from EX and SRX devices. Uses VT100 screen-buffer rendering for proper in-place display.

#### **VT100 Terminal Emulation**
Added ANSI escape stripping and a minimal VT100 screen-buffer renderer for device command output. Stream-mode commands (ping, traceroute) have ANSI codes stripped automatically. Screen-mode commands (top, monitor interface) are rendered through a virtual terminal buffer.

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### 2. IMPROVEMENTS

#### **Non-Blocking Device Utilities**
All `mistapi.device_utils` functions now return immediately. The HTTP trigger and WebSocket streaming run in background threads, allowing your code to continue executing while data is collected.

**UtilResponse Object:**
| Method/Property | Description |
|-----------------|-------------|
| `.ws_data` | List of processed messages |
| `.done` | `True` if data collection is complete |
| `.wait(timeout)` | Block until complete, returns self |
| `.receive()` | Generator yielding messages as they arrive |
| `.disconnect()` | Stop the WebSocket connection early |
| `await response` | Async-friendly wait (non-blocking event loop) |

**Example Usage:**
```python
from mistapi.device_utils import ex

# Non-blocking - returns immediately, data collected in background
response = ex.ping(apisession, site_id, device_id, host="8.8.8.8")
do_other_work()  # Can do other things while waiting
response.wait()  # Block when ready to collect results
print(response.ws_data)

# Generator style - process messages as they arrive
for msg in response.receive():
    print(msg)

# Async-friendly - doesn't block the event loop
await response
```

#### **Binary WebSocket Frame Support**
`_MistWebsocket._handle_message()` now handles binary frames (strips null bytes, decodes UTF-8 with replacement characters).

#### **Trigger-Only Commands Run Synchronously**
Fire-and-forget device commands (e.g., `clearMacTable`, `clearBpduError`, `clearHitCount`) that don't require a WebSocket stream now run the API trigger synchronously, ensuring `trigger_api_response` is immediately available on the returned `UtilResponse`.

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### 3. BUG FIXES

- Fixed double-space typo in API token privilege mismatch error message
- Fixed `first_message_timeout` timer stop to check timer is active before stopping

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### 4. DEPENDENCIES

- Added `sshkeyboard>=2.3.1` (for `interactiveShell()`)

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