Static Routes with nexthop non-functional for private gateways

[bhouse-nexthop]

Description

Static routes were only being added to the main routing table, but policy-based routing (PBR) is active on VPC routers. This caused traffic coming in from specific interfaces to not find the static routes, as they use interface-specific routing tables (Table_ethX).

This fix:

• Adds a helper method to find which interface a gateway belongs to by matching the gateway IP against configured interface subnets
• Modifies route add/delete operations to update both the main table and the appropriate interface-specific PBR table
• Uses existing CsAddress databag metadata to avoid OS queries
• Handles both add and revoke operations for proper cleanup
• Adds comprehensive logging for troubleshooting

Fixes #12857

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• Build/CI
• Test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

See #12857 for the tested scenario

How did you try to break this feature and the system with this change?

[boring-cyborg]

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