[Nexthop] Fix baseline stats retrieval for ACL queue per host tests

[vvasavada-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

The test expects to run as follows:

1. Read the counter before (baseline) -> statBefore
2. Send packets
3. Read the counter after -> statAfter
4. Check statAfter - statBefore is expected value

Problem:

The HW agent process polls the ACL counters and pushes them to the SW agent every 1s. Until the first push lands, the SW agent's cached stats map is empty - and the old getAclInOutPackets couldn't distinguish "no HwSwitch has reported this coutner yet" from "the counter reports 0". It returned 0 for both.

Across a warm boot the hardware counter keeps it pre-warmboot value (20). But if the baseline is read before the first stats push arrives, statBefore gets 0 instead of 20, so the delta comes out as the full accumulated count:

  08:57:07.837500 AgentQueuePerHostTests.cpp:231] Verify class id for 1.0.0.14 ...   <- baseline read here
  08:57:07.877734 MultiSwitchThriftHandler.cpp:378] Got stats event from switchIndex 0  <- stats polled
  08:57:10.840604 AgentQueuePerHostTests.cpp:349]  Acl stats : 25                    <- 25-0 != 5, fails 30s
  08:57:41.849150 AgentQueuePerHostTests.cpp:349]  Acl stats : 30                    <- next sub-phase passes

Since statBefore is not in WITH_RETRIES loop, it stays 0 for full 30s until timeout.

Fix:

getAclInOutPacketsIf returns nullopt when no HwSwitch has reported the counter, and 0 only when 0 is genuinely reported. getLatestAclInOutPackets wraps it to wait until the counter is present, mirroring how getLatestPortStats waits for port stats. Only the baseline snapshot switch to it - getAclInOutPackets keeps its existing absent-is-0 behavior, so other tests are unchanged.

As a side fix, also make sure HwFb303Counter::cumulativeValue is explicitly inited to 0.

Test Plan

[ PASSED ] cold_boot.AgentQueuePerHostL2Test.VerifyHostToQueueMappingClassID (17887 ms)
[ PASSED ] warm_boot.AgentQueuePerHostL2Test.VerifyHostToQueueMappingClassID (8544 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsAfterResolve (20091 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsAfterResolve (12629 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsAfterResolveBlock (16187 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsAfterResolveBlock (4755 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsWithResolve (21140 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsWithResolve (12542 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsWithResolveBlock (16035 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.VerifyHostToQueueMappingClassIDsWithResolveBlock (4705 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.VerifyTtldCounter (20290 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.VerifyTtldCounter (8604 ms)
[ PASSED ] cold_boot.AgentQueuePerHostTest.RemovePendingNeighborDoesNotCrash (16035 ms)
[ PASSED ] warm_boot.AgentQueuePerHostTest.RemovePendingNeighborDoesNotCrash (4700 ms)
[ PASSED ] cold_boot.AgentQueuePerHostRouteTest.VerifyHostToQueueMappingClassID (33005 ms)
[ PASSED ] warm_boot.AgentQueuePerHostRouteTest.VerifyHostToQueueMappingClassID (13857 ms)
[ PASSED ] cold_boot.AgentQueuePerHostRouteTest.VerifyHostToQueueMappingClassIDBlock (29143 ms)
[ PASSED ] warm_boot.AgentQueuePerHostRouteTest.VerifyHostToQueueMappingClassIDBlock (9886 ms)
Summary:
   PASSED : 18
   FAILED : 0
   SKIPPED : 0
   TIMEOUT : 0