fix missing main route table for the default vpc

[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 1959a1e

Fix route table sync for VPCs with implicit main route table. Include the main route table of the VPC in the rtbs map in pkg/controller/vpc.go.

🤖 Generated by Copilot at 1959a1e

rtbs map expands
to include main route table
cutting sync errors

HOW

🤖 Generated by Copilot at 1959a1e

• Ensure main route table is always in rtbs map (link)

[github-actions]

• The addition of rtbs[util.MainRouteTable] = nil in line 277 might cause unexpected behavior. It is recommended to add a comment explaining the reason for this change and how it affects the code logic.
• The error handling in lines 284-287 could be improved. Instead of checking if the error is not found, it would be better to check if the error is found and handle it accordingly. This will prevent unexpected errors from being ignored.
• The function c.getRouteTablesByVpc(vpc) in line 275 should have a comment explaining what it does and how it affects the code logic. This will make it easier for other developers to understand the code.
• The variable name joinSubnet in line 280 is not descriptive enough. It is recommended to use a more descriptive name that reflects its purpose in the code.
• The function handleAddOrUpdateVpc(key string) error in line 269 should have a comment explaining what it does and how it affects the code logic. This will make it easier for other developers to understand the code.