Add STP loop guard support

[salahadell]

Summary

Add STP Loop Guard support to sonic-stp and wire it through manager, runtime state handling, and APP_DB sync.

Motivation

When expected BPDUs are lost on non-designated ports, the port should not transition unsafely toward forwarding, potentially creating Layer-2 loops, and broadcast storms.
Loop Guard keeps the port in loop-inconsistent state until recovery conditions are met.

Changes

• Added loop-guard config field in IPC/config path.
• Added loop-guard mask and runtime state in STP global/port data.
• Added loop-guard activation on message-age expiry for eligible non-designated ports.
• Added BPDU-based recovery logic to clear loop-inconsistent state.
• Added APP_DB sync for loop-guard active status.
• Added debug visibility for loop-guard config/state.
• Added disable-path cleanup so stale loop-guard state is cleared and STP re-evaluation runs immediately.

Behavior

• With loop guard enabled, BPDU loss on protected non-designated ports enters LOOP-INC.
• Receiving valid BPDU clears loop-guard active state and normal STP processing continues.
• Disabling loop guard while port is LOOP-INC clears stale state and triggers immediate STP state re-evaluation.

Files Updated

• include/stp.h
• include/stp_ipc.h
• include/stp_dbsync.h
• stp/stp.c
• stp/stp_mgr.c
• stp/stp_util.c
• stp/stp_data.c
• stp/stp_debug.c
• stpsync/stp_sync.cpp

Validation

• Interop test with SONiC DUT connected to a Cisco switch and SONiC peers.
• Enabled loop guard on SONiC DUT test interface.
• On Cisco side, enabled BPDU filter to stop BPDU transmission.
• Observed SONiC port moved to LOOP-INC when BPDUs stopped.
• After BPDU transmission resumed, SONiC port recovered through normal STP behavior.
• Confirmed loop-guard state updates were reflected in APP_DB.

Related PRs

• sonic-swss: Adding loop guard support sonic-swss#4517
• sonic-utilities: Add loop guard CLI commands sonic-utilities#4495

[mssonicbld]

/azp run

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