Fix system-health crash on VS platform due to missing metadata file

[yxieca]

Description

The VS chassis platform module (sonic_platform/chassis.py) raises FileNotFoundError when /etc/sonic/vs_chassis_metadata.json does not exist. This file is only present on VS chassis (T2-VOQ) setups introduced in #18512, not on standalone VS platforms (e.g., ToR used in KVM testbeds).

Impact

The crash prevents system-health.service from running, which means SYSTEM_READY|SYSTEM_STATE is never set in STATE_DB. This blocks any daemon that waits for system-ready — specifically hsflowd in the sflow container, which loops in waitConfig/getSystemReady() forever, never reads CONFIG_DB, and never generates /etc/hsflowd.auto.

This causes all sflow KVM tests to fail on trixie images with:

Failed: hsflowd failed to initialize collector(s) ['20.1.1.2'] within 240 seconds

The issue is currently masked in CI because sflow tests are blanket-skipped via tests_mark_conditions.yaml (sonic-mgmt#21701).

Fix

Return an empty metadata dict when the file doesn't exist instead of raising an exception. The chassis methods that consume metadata (get_supervisor_slot, get_linecard_slot, get_my_slot) already raise KeyError/ValueError for missing fields, so the behavior is correct for chassis setups.

Testing

• Verified on KVM testbed: after manually setting SYSTEM_READY (the workaround), hsflowd initializes correctly and sflow tests pass
• The fix ensures system-health.service starts successfully on standalone VS, which sets SYSTEM_READY automatically

Fixes #26429

[mssonicbld]

/azp run Azure.sonic-buildimage

[Copilot]

Pull request overview

Fixes a startup crash in the VS sonic_platform chassis implementation by making metadata loading tolerant of a missing /etc/sonic/vs_chassis_metadata.json, which prevents system-health.service from failing on standalone VS platforms.

Changes:

• Remove the FileNotFoundError thrown when the VS chassis metadata file is absent.
• Allow chassis initialization to proceed with an empty metadata dictionary when the file is missing.

[yxieca]

Closing — need to build and test locally before re-raising. The initial fix was incomplete (didn't preserve FileNotFoundError for T2-VOQ chassis). Will reopen with proper local testing.