fix(snmp): bundle pysnmp in image and port poller to the asyncio HLAPI

The SNMP plugin ships in the image but logged "pysnmp not installed — SNMP
polling disabled" on every poll, so polling never worked. Two coupled defects:

1. The Dockerfile installed only `.[ansible]`, so the `snmp` extra (pysnmp)
   was never bundled even though the plugin is first-party and shipped.
2. poller.py used the synchronous pysnmp HLAPI (`next(getCmd(...))`), which
   pysnmp-lextudio 6.x removed — it's asyncio-only now — so even with the dep
   present, polling would have thrown and silently returned nothing. The 5.x
   line that still has the sync API isn't safe on the image's Python 3.13.

Fix:
- Dockerfile: install `.[ansible,snmp]`.
- poller.py: `poll_device_sync` → `async def poll_device` on the asyncio HLAPI,
  with a dual-version import (pysnmp 7.x `pysnmp.hlapi.v3arch.asyncio`/`get_cmd`
  + async `UdpTransportTarget.create`; pysnmp-lextudio 6.2.x
  `pysnmp.hlapi.asyncio`/`getCmd` + direct `UdpTransportTarget`) so a dependency
  bump can't silently re-break it.
- scheduler.py: await poll_device directly; drop the run_in_executor wrapper
  and the now-unused asyncio import.
- Add tests/plugins/snmp/test_poller.py covering the version→mpModel mapping,
  that the poller is a coroutine, and the graceful no-pysnmp path.

Note: CI confirms import/load and the no-pysnmp path, but has no SNMP target —
live polling against real devices is verified after deploy.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

plugins/snmp/scheduler.py

@@ -1,6 +1,5 @@
 # plugins/snmp/scheduler.py
 from __future__ import annotations
-import asyncio
 import logging
 from typing import TYPE_CHECKING
 
@@ -27,15 +26,13 @@ def make_poll_task(app: "Quart") -> ScheduledTask:
 
 
 async def _do_poll(app: "Quart") -> None:
-    from .poller import poll_device_sync
+    from .poller import poll_device
     from steward.core.alerts import record_metric
 
     devices: list[dict] = app.config["PLUGINS"]["snmp"].get("devices", [])
     if not devices:
         return
 
-    loop = asyncio.get_event_loop()
-
     async with app.db_sessionmaker() as session:
         async with session.begin():
             for device in devices:
@@ -52,11 +49,7 @@ async def _do_poll(app: "Quart") -> None:
                     continue
 
                 try:
-                    readings = await loop.run_in_executor(
-                        None,
-                        poll_device_sync,
-                        host, port, community, version, oids,
-                    )
+                    readings = await poll_device(host, port, community, version, oids)
                 except Exception:
                     logger.exception("SNMP poll failed for device %s (%s)", name, host)
                     continue