Fixed bulk SNMP operations by making the protocol version configurable

[cafe-jun]

Closes #2150

Problem

SNMPConnector hardcodes credentials.V1, so every request goes out with the SNMP version field set to 0 (SNMPv1):

https://github.com/thingsboard/thingsboard-gateway/blob/master/thingsboard_gateway/connectors/snmp/snmp_connector.py#L171-L173

GETBULK was introduced in SNMPv2c. Carried inside an SNMPv1 message it is discarded by the agent, and the client times out — the TimeoutError reported in #2150.

This is not limited to bulkwalk. Three of the eleven advertised methods are built on GETBULK and are therefore unusable:

• bulkget
• bulkwalk
• bulktable

In puresnmp, V2C subclasses V1 and overrides only mpm (0 → 1), which is the version field in the packet. The credential class alone decides whether bulk operations can work.

Reproduction

Against a local snmpsim agent (community public), walking 1.3.6.1.2.1.2.2.1.2 (3 entries):

credentials	get	bulkwalk
V1 (current)	OK	Timeout: 3 second timeout exceeded on UDP transport.
V2C	OK	OK — 3 rows

Change

Adds an optional per-device version key accepting v1 or v2c, defaulting to v2c. SNMPv1-only agents stay reachable by setting it explicitly. An unknown value logs a warning and falls back to the default rather than breaking the poll.

The key is also added to the shipped config/snmp.json example so it is discoverable.

Verification

Exercised through SNMPConnector.__process_methods against the same agent:

device config	resolved version	method	result
no version key	v2c	bulkwalk	OK — 3 rows
"version": "v1"	v1	bulkwalk	times out (previous behaviour, now opt-in)
"version": "v1"	v1	get	OK — no regression for v1-only agents
"version": "V2C"	v2c	bulkwalk	OK — case normalised
"version": "v9"	falls back	bulkwalk	OK — warning logged

On the default value

I defaulted to v2c because the connector advertises bulk methods that cannot work under v1, and puresnmp itself emits UserWarning: Experimental SNMPv1 support. This does change behaviour for anyone relying on the implicit v1, so if you would rather keep v1 as the default and have users opt into v2c, say the word and I will flip it — the change is one constant.

Notes

• No tests added: there is currently no SNMP coverage under tests/. Happy to add unit tests for the version resolution if you want them in this PR.
• Out of scope, but noticed while working on this: config/snmp.json shows a per-datapoint "community" override, while the connector only reads device["community"]. I left it alone to keep this change focused.
• v3 is not included since it needs auth/priv parameters and a wider config surface.

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