docs: document VEG remaining Ready with zero BFD sessions

[changluyi]

Summary

• Add bilingual troubleshooting documentation for the case where a VPC Egress Gateway remains Ready while bfdd-control status reports There are 0 sessions:.
• Explain that the existing exec probe returns success for an empty session table, so kubelet does not restart bfdd.
• Recommend upgrading affected ACP 4.3.0-4.3.2 clusters to ACP 4.3.3 or later, with a temporary liveness-probe workaround only when an immediate upgrade is not possible.

Verification

• English SHA256 matches the Chinese sourceSHA.
• English and Chinese documents use the standard Issue / Root Cause / Resolution / Diagnostic Steps structure.
• CPU and BFD interval tuning content has been removed from this single-issue article.
• Markdown whitespace checks pass and no kubectl patch commands are used.