fix vpc already delete while delete policy route

[bobz965]

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What type of this PR

Examples of user facing changes:

• Bug fixes

Which issue(s) this PR fixes:

Fixes #2877

WHAT

🤖 Generated by Copilot at 3a7e127

Improve policy route handling for custom VPC subnets. Prevent errors and unnecessary operations in subnet.go when deleting VPCs or subnets.

🤖 Generated by Copilot at 3a7e127

Oh we are the coders of the policy routes
We work with the VPCs and the subnets too
We check for the errors and the deletions
And we pull and we push and we merge on the count of three

HOW

🤖 Generated by Copilot at 3a7e127

• Add a check for logical router existence before adding or deleting policy routes for U2O interconnection (link, link) in pkg/controller/subnet.go