[Nexthop] Fix SIGABRT in h/w agent on unsupported packet RX reason

[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

SaiHostifManager::packetReasonToHostifTrap() threw an FbossError for any packet RX reason it could not map to a SAI hostif trap on the current ASIC/SDK build (e.g. L3_SLOW_PATH, MPLS_UNKNOWN_LABEL). When such a reason arrives in control plane delta, the exception escapes the oper-delta thread in SaiSwitch::stateChangedImplLocked, aborting the h/w agent with SIGABRT.

The software agent's config validation layer (isValidRxReasonToQueue) cannot fully gate these reasons, because some are gated by compile-time SAI SDK macros (SAI_API_VERSION, BRCM_SAI_SDK_DNX_GTE_11_0, etc.) that are visible only in the SAI layer, not in the platform-agnostic agent code.

Fix

Make the SAI hostif trap mapping non-fatal, matching the established skip-with-diagnostic pattern already used throughout the SAI managers (and already used for UNMATCHED in this same function):

• packetReasonToHostifTrap() and makeHostifTrapAttributes() now return std::optional.
• An unsupported/unmappable RX reason logs XLOG(ERR) and returns std::nullopt instead of throwing.
• Callers skip programming the trap when no attributes are produced, so the agent stays up.

Test Plan

Before the fix: HostifManagerTest.unsupportedHostifTrapSkipped fails
After the fix: HostifManagerTest.unsupportedHostifTrapSkipped passes.