snmpagent: Implement an asyncio-based SNMP trap infrastructure supporting link up/down, PSU, and fan traps

[Griffin-micas]

- What I did

• Added an asyncio-based SNMP Trap infrastructure (TrapInfra) to SONiC SNMP agent.
• Implemented native support for Link Up/Down and PSU / FAN SNMP traps.
• Generated traps as AgentX Notify PDUs carrying snmpTrapOID.0 and business varbinds, while keeping the final SNMP notification version and behavior (trap / inform) fully controlled by snmpd configuration.
• Ensured traps are triggered only on semantic state changes, preventing unnecessary trap storms.

- How I did it

Introduced a generic TrapInfra layer that:

• Subscribes to Redis keyspace notifications (multi-DB, multi-instance supported).
• Dispatches events to corresponding trap handlers using asyncio tasks.
• Wraps handler output into AgentX NotifyPDUs and forwards them to snmpd.

Implemented dedicated trap handlers:

1. Link Up/Down traps

• Trap OIDs: linkUp (1.3.6.1.6.3.1.1.5.4), linkDown (1.3.6.1.6.3.1.1.5.3)
• Varbinds: ifIndex, ifAdminStatus, ifOperStatus
• Interface index resolved via mibs.get_index_from_str(ifName)

2. PSU / FAN traps

• FAN Trap OID: 1.3.6.1.4.1.9.9.117.1.4.1.1.1
• PSU Trap OID: 1.3.6.1.4.1.9.9.117.1.1.2.1.2
• Index values derived deterministically from object names (PSU, FANTRAY)

All handlers compute varbinds based on state transitions, not raw field updates.
TrapInfra lifecycle is fully asyncio-managed and supports graceful shutdown.

- How to verify it

1. Configure SNMP trap destinations in snmpd.
2. Start snmpagent container and confirm TrapInfra subscriptions are active.
3. Trigger events on DUT:
   Link state change (config interface shutdown/startup)
   PSU or FAN state change (hardware event or simulated Redis update)
4. Verify:
   SNMP notifications are sent as v1/v2c Notifications.
   snmpTrapOID.0 matches the expected trap OID.
   Varbind values (ifIndex, ifOperStatus, PSU/FAN status) are correct.
   No duplicate traps are generated without actual semantic state changes.

- Description for the changelog

Add asyncio-based SNMP trap infrastructure with native support for link and PSU/FAN notifications.

Which release branch to backport (provide reason below if selected)

• 202511

Reason for backport:
This change provides standard SNMP trap support (link, PSU, and FAN), which is required for basic device monitoring and NMS integration.
The implementation is self-contained, does not change existing SNMP behavior unless traps are explicitly configured, and is safe to include in 202511 releases.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Griffin-micas / name: Griffin Gao (5449e15)

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[Griffin-micas]

@qiluo-msft Hi Qiluo, could you please take a look at this PR when you have time? Thanks!