[Nexthop] warm-reboot: disable FLOW_CNT_TRAP before warm shutdown

[wren-nexthop]

What I did

Resolves #4753

Disable the FLOW_CNT_TRAP flex counter group before warm-boot shutdown, and roll the change back if the reboot aborts before kexec.

With trap flow counters enabled, orchagent binds a SAI counter to every HOSTIF trap via SAI_HOSTIF_TRAP_ATTR_COUNTER_ID. After warm boot, syncd replays that attribute during APPLY_VIEW and Broadcom SAI fails with BCM_E_NOT_FOUND, surfacing as SAI_STATUS_ITEM_NOT_FOUND; syncd enters shutdown-wait and orchagent crash-loops. Disabling the group before the warm shutdown lets CoppOrch unbind and clear every trap counter through SAI while the handles are still valid.

How I did it

• If FLOW_CNT_TRAP is enabled, disable it in CONFIG_DB before the warm shutdown and wait for COUNTERS_TRAP_NAME_MAP to drain.
• Write a marker file under /host/warmboot (flow_cnt_trap_need_re_enable) so the warmboot-finalizer knows to re-enable the group after a successful reconcile. The marker is only written when re-enable is actually needed, so its absence is the no-op signal.
• If the warm-reboot aborts before kexec, clear_boot re-enables the counter group and removes the marker.

The finalizer side (re-enable after reconcile) is the companion sonic-buildimage PR: sonic-net/sonic-buildimage#28896

How to verify it

1. Enable FLOW_CNT_TRAP in CONFIG_DB and config reload.
2. sudo warm-reboot.
3. After kexec: links up, no container crash-loops, warmboot finalizer completes.
4. show flowcnt-trap stats works once the counter group is re-enabled by the finalizer.

Verified on Broadcom Tomahawk-based platforms together with the companion finalizer change; also verified the abort path (interrupted warm-reboot re-enables the group and removes the marker).

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).