[clear] Add 'sonic-clear pfcwdstats' to clear PFC watchdog counters

[pinky-nexthop]

Why I did it

There is currently no CLI to clear PFC watchdog statistics. Clearing them is needed when troubleshooting PFC storms — stale counts from a prior event make it hard to tell whether a storm is currently ongoing. show pfcwd stats displays the counters, but there is no corresponding clear command.

Addresses the PFC-watchdog-counter-clear item in #4595.

How I did it

• scripts/pfcwdstat: new tool that zeroes the PFC_WD_QUEUE_STATS_* fields in COUNTERS_DB, following the standard counter-clear pattern (pfcstat -c, queuestat -c, …). It enumerates queues via COUNTERS_QUEUE_NAME_MAP — the same source show pfcwd stats uses — so stale stats from queues that are no longer actively monitored (STATUS=N/A) are also cleared.
• Queues with an active storm (PFC_WD_STATUS == 'stormed') are skipped, so the in-flight DEADLOCK_DETECTED/DEADLOCK_RESTORED pair is not split (which would otherwise leave RESTORED > DETECTED once the storm ends).
• clear/main.py: adds sonic-clear pfcwdstats, invoking pfcwdstat -c.
• setup.py: installs scripts/pfcwdstat.
• Unit tests for the clear command and the pfcwdstat clear logic (including the stale-stats and active-storm cases).

How to verify it

admin@sonic:~$ show pfcwd stats
        QUEUE       STATUS    STORM DETECTED/RESTORED    TX OK/DROP    RX OK/DROP    TX LAST OK/DROP    RX LAST OK/DROP
-------------  -----------  -------------------------  ------------  ------------  -----------------  -----------------
Ethernet192:3  operational                        1/1           0/0           0/0                0/0                0/0

admin@sonic:~$ sonic-clear pfcwdstats
PFC Watchdog counters cleared.

admin@sonic:~$ show pfcwd stats
  QUEUE    STATUS    STORM DETECTED/RESTORED    TX OK/DROP    RX OK/DROP    TX LAST OK/DROP    RX LAST OK/DROP
-------  --------  -------------------------  ------------  ------------  -----------------  -----------------

pytest tests/pfcwd_test.py::TestPfcwdstatClear tests/clear_test.py -k pfcwd

Manually: show pfcwd stats to see counts, sonic-clear pfcwdstats to clear them; an actively storming queue is left untouched (and reported as skipped) until the storm ends.

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

Description for the changelog

Add sonic-clear pfcwdstats to clear PFC watchdog counters.

[mssonicbld]

/azp run

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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