e2e: add RFC-18 flex-algo end-to-end tests (RFC-18, PR 4/4)

[ben-malbeclabs]

Summary of Changes

• Adds five new end-to-end test functions covering the full RFC-18 operator workflow:
  • TestE2E_FlexAlgo_TopologyLifecycle — create a topology, tag a link, attempt delete (expect error), clear the topology, delete successfully
  • TestE2E_FlexAlgo_TopologyFilter — verify doublezero link list --topology <name> returns correct links before and after tagging; includes --topology default (untagged) and named topology cases
  • TestE2E_FlexAlgo_UnicastDrained — set and clear --unicast-drained on a link; poll the Go SDK to confirm link_flags bit 0 transitions correctly
  • TestE2E_FlexAlgo_TenantIncludeTopologies — set and clear --include-topologies on a tenant; poll SDK for field changes (lightweight devnet, activator disabled)
  • TestE2E_FlexAlgo_Migrate — clear topologies from a link, run doublezero-admin migrate flex-algo --dry-run (verify "DRY RUN" output and count), run live migrate, poll SDK to confirm link is re-tagged
• Adds TestE2E_FlexAlgo_Controller — exercises controller config generation end-to-end with a live cEOS device
• Updates TestE2E_LinkOnchainAllocation to create UNICAST-DEFAULT topology before link activation (now required by the program)
• Adds --features-config plumbing to the devnet controller container and devnet.WithFeaturesConfig helper

Stacked PRs: This is PR 4/4. Depends on #3474 (smartcontract), #3475 (activator), and #3476 (controller). Merge last.

RFC: rfcs/rfc18-link-classification-flex-algo.md

Diff Breakdown

Category	Files	Lines (+/-)	Net
Tests	6	+1,151 / -0	+1,151
Scaffolding	7	+60 / -7	+53

Entirely test additions plus the devnet infrastructure to support them.

Key files (click to expand)

• e2e/topology_filter_test.go — 368 lines; validates --topology filter across create/tag/untag cycles
• e2e/topology_lifecycle_test.go — 222 lines; verifies safe-delete enforcement (can't delete while links reference topology)
• e2e/flex_algo_test.go — 231 lines; controller config generation test against live cEOS
• e2e/topology_migrate_test.go — 191 lines; dry-run and live migrate with SDK polling
• e2e/tenant_topology_test.go — 139 lines; lightweight devnet for tenant field changes
• e2e/internal/devnet/controller.go — WithFeaturesConfig devnet option for passing features.yaml to the controller container

Testing Verification

• All six new test functions compile and are wired into the e2e suite
• TestE2E_LinkOnchainAllocation updated to create UNICAST-DEFAULT first — verifies the existing link allocation flow still works after the activation constraint was added
• Tests use Go SDK polling (not CLI output alone) to confirm onchain state transitions for link_flags, link_topologies, and include_topologies