UNICAST-DRAINED system constant in controller templates

[ben-malbeclabs]

The controller MUST always define administrative-group alias UNICAST-DRAINED group 1 in the router traffic-engineering block on all devices when enabled: true. This is a system constant — not derived from any TopologyInfo PDA.

Every include-any flex-algo definition MUST include exclude 1 (the UNICAST-DRAINED bit) unconditionally. This is structural — not conditional on any link being drained.

When link.unicast_drained = true, the controller appends UNICAST-DRAINED to the interface's traffic-engineering administrative-group alongside permanent topology tags. When false, only permanent topology tags are rendered.

Milestone: RFC-18: Controller — Topology Config
RFC: #3288

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Files affected:

• controlplane/controller/internal/controller/ (device templates)

Size: Small

Tests:

• UNICAST-DRAINED group 1 alias always present in router traffic-engineering when enabled: true, regardless of drain state
• exclude 1 always present in all include-any flex-algo definitions
• Link with unicast_drained = true and link_topologies[0] = UNICAST-DEFAULT produces traffic-engineering administrative-group UNICAST-DEFAULT UNICAST-DRAINED
• Link with unicast_drained = false produces only permanent topology tags
• Permanent topology tags never modified when drain state changes

Blockers: #3351 (IS-IS TE templates)