Block C migration miss: discovery.py:_snmp_get still uses pysnmp 6.x API

[commitconfirm]

Summary

controller/app/discovery.py:_snmp_get() raises an ImportError at
first call:

cannot import name 'getCmd' from 'pysnmp.hlapi.asyncio'

The project is correctly pinned to pysnmp==7.1.24 in
controller/requirements.txt. The rest of the SNMP code path
(controller/app/snmp_collector.py, used by all Block C collectors)
was migrated to the pysnmp 7.x API as a v1.0 prerequisite. This one
function in discovery.py was missed because it pre-dates the SNMP
collector refactor and was never refactored to use it.

Root cause

pysnmp 7.x removed the 6.x camelCase API names. discovery.py:_snmp_get
still uses them:

# controller/app/discovery.py, ~line 773
from pysnmp.hlapi.asyncio import (
    CommunityData, ContextData, ObjectIdentity, ObjectType,
    SnmpEngine, UdpTransportTarget, getCmd,        # <-- 6.x camelCase
)
transport = UdpTransportTarget((ip, 161), ...)    # <-- 6.x sync ctor
...
await getCmd(...)                                  # <-- 6.x name

The 7.x equivalents (already used throughout snmp_collector.py):

• getCmd → get_cmd (snake_case)
• UdpTransportTarget(...) → await UdpTransportTarget.create(...) (async factory)

The import path pysnmp.hlapi.asyncio itself is still valid in 7.x (it is
the backward-compat alias for pysnmp.hlapi.v3arch.asyncio); only the
function names and the transport constructor changed.

Impact

SNMP discovery probes fail before any UDP packet is sent. Sweep IPs that
should have surfaced sysDescr / sysObjectID / sysName via SNMP fall
through to the non-SNMP classification path. Onboarding-time classifier
discrimination loses its strongest signal.

No impact on Block C collectors (ARP/MAC/LLDP/routes) - those use
snmp_collector.py, which is correctly on the 7.x API.

Fix

Three-line change in _snmp_get:

1. Import get_cmd instead of getCmd.
2. await UdpTransportTarget.create((ip, 161), timeout=3, retries=1)
   instead of the sync constructor.
3. Call await get_cmd(...) instead of await getCmd(...).

Mechanical pattern - mirrors what snmp_collector.py lines 40-49 and
~380, ~499 already do.

Related cleanup (worth bundling)

_snmp_get instantiates a fresh SnmpEngine() per call. This is the
exact UDP-socket-leak pattern the LEAK-FIX (mnm commit ebbfef8)
removed from snmp_collector.py by introducing a shared
_get_engine() singleton. The remaining per-call leak in _snmp_get
is slower than the pre-LEAK-FIX rate (because discovery is operator-
triggered, not continuous), but it's the same class of bug. The fix
should switch _snmp_get to use snmp_collector._get_engine() rather
than creating its own SnmpEngine().

Acceptance criteria

• cannot import name 'getCmd' no longer appears in controller logs
  during SNMP discovery.
• Lab devices with valid SNMP credentials surface sysDescr /
  sysObjectID / sysName / sysContact / sysLocation /
  sysUpTime via the discovery path again.
• FD count on the controller is flat across N discovery probes
  (LEAK-FIX-pattern regression check; pattern in
  tests/unit/test_snmp_collector.py::test_walk_table_does_not_leak_fds_real_socket_path).
• No regressions in snmp_collector.py consumers (ARP/MAC/LLDP/routes
  Block C collectors).

Lesson (for CLAUDE.md)

When migrating a dependency with API breaking changes, grep the
entire codebase for the old API names, not just the high-traffic
modules. The Block C migration covered snmp_collector.py (the SNMP
abstraction layer) and left discovery.py:_snmp_get (a pre-existing
inline SNMP caller) as a straggler. Two greps would have caught it
at migration time:

grep -rn "getCmd\\|bulkCmd\\|nextCmd\\|setCmd" .
grep -rn "UdpTransportTarget(" .   # 6.x sync constructor

Both grep results should be exhaustively reviewed during any pysnmp
(or other dep-with-major-version-restructure) upgrade.

References

• pysnmp 7.x migration notes: https://docs.lextudio.com/pysnmp/
  (snake_case rename + v3arch.asyncio reorganization)
• snmp_collector.py migration commit: 6b548e2 chore: migrate pysnmp-lextudio to pysnmp 7.x
• LEAK-FIX commit (related shared-engine pattern): ebbfef8 fix(controller): SnmpEngine FD leak in snmp_collector