bgpd: preserve cli configured VPN export RT on router-id update

[kaffarell]

rt vpn both is stored internally as separate import and export RT lists. On router-id updates, bgpd re-derives the VPN RD/RT and overwrites the export RT with an RT derived from the current export RD.

This broke configurations such as:

 rd vpn export 65000:6
 rt vpn both 65000:100

After a router-id update, the import RT stayed 65000:100, but the export RT was replaced with 65000:6. The running config then appeared as split import/export RTs and VPN import matching no longer worked as configured.

Add a "CLI set" flag for the VPN export RT, set/clear it from the RT CLI, and skip automatic export RT derivation when the export RT was explicitly configured. Also do the same thing to the VRF-to-VRF auto RT setup and last-unimport cleanup, so explicit (cli-set) RT/RD values survive removing and re-adding import vrf.

Note that the router-id import-vrf RT refresh still removes only the first RT value before merging the current export RT list. This behavior already existed beforehand (the old code also removed ecom->val only) so I left it unchanged.

Fixes: #22579

[greptile-apps]

Greptile Summary

This PR preserves manually configured VPN export RTs across bgpd refresh paths. The main changes are:

• Adds an export RT CLI-owned flag to VPN policy state.
• Sets and clears that flag from rt vpn export and rt vpn both CLI handling.
• Skips automatic export RT re-derivation when the RT was configured by CLI.
• Preserves CLI-owned export RT/RD values when the last import vrf is removed.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
bgpd/bgp_mplsvpn.c	Adds CLI-owned RT checks to router-id refresh, import-vrf setup, and last-unimport cleanup.
bgpd/bgp_vty.c	Sets and clears the new export RT ownership flag from RT CLI commands.
bgpd/bgpd.h	Defines the new VPN policy flag used to mark CLI-configured export RTs.

_{Reviews (1): Last reviewed commit: "bgpd: preserve cli configured VPN export..." | Re-trigger Greptile}

[routingrocks]

LGTM