client: fix OnClose route deletion to scope by peer and skip NoInstall

[packethog]

Summary

• Cherry-pick of client: fix OnClose route deletion to scope by peer and skip NoInstall #3116 onto the hotfix/client/v0.8.11 branch (without CHANGELOG.md to avoid merge conflict)
• Fix BGP OnClose deleting routes from all peers instead of only the closing peer — a multicast peer teardown was wiping unicast routes and breaking connectivity
• Scope route deletion to only routes whose gateway matches the closing peer's address
• Skip route deletion entirely for NoInstall peers (multicast) since they never install kernel routes

Testing Verification

• Existing OnClose test updated to include routes from two different peers, verifying only the closing peer's routes are deleted
• New TestClient_BGPPlugin_OnCloseSkipsRouteDeletionWhenNoInstall verifies that NoInstall peers do not query or delete routes