libovsdb: ignore not found error when listing objects with a filter

[zhangzujian]

• Make sure you have followed Kube-OVN Code Style.

What type of this PR

• Bug fixes

Which issue(s) this PR fixes:

Fixes #(issue-number)

WHAT

🤖 Generated by Copilot at 63b99a0

Improved error handling and client usage in ovn-nb-logical_router_policy.go, ovn-nb-logical_router_route.go, and ovn-nb-nat.go. The changes make the code more robust and consistent when querying OVN database objects.

🤖 Generated by Copilot at 63b99a0

No more errors, no more lies
We filter out the missing ones
With libovsdb we defy
The ovn-nb database of doom

HOW

🤖 Generated by Copilot at 63b99a0

• Add errors and client packages to import statements in ovn-nb-logical_router_policy.go, ovn-nb-logical_router_route.go, and ovn-nb-nat.go to handle client.ErrNotFound error and interact with OVN database (link, link, link)
• Skip non-existent logical router policies, routes, and NATs in listLogicalRouterPoliciesByFilter, listLogicalRouterStaticRoutesByFilter, and listLogicalRouterNatByFilter functions by checking for client.ErrNotFound error and continuing the loop (link, link, link)