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

Dockerfile

@@ -19,8 +19,10 @@ WORKDIR /app
 
 COPY pyproject.toml .
 COPY steward/ steward/
-# .[ansible] pulls the full Ansible package so the playbook runner works in-image.
-RUN pip install --no-cache-dir '.[ansible]'
+# Bundle the extras whose first-party plugins ship in the image: [ansible] for
+# the playbook runner, [snmp] (pysnmp) for the SNMP poller. Without them those
+# bundled plugins load but silently no-op for want of their runtime dependency.
+RUN pip install --no-cache-dir '.[ansible,snmp]'
 
 COPY alembic.ini .
 # First-party plugins ship inside the image (bundled root at /app/plugins).