fix(test): wait for iperf server migration before continuity checks

[nevermarine]

What & why

The post-upgrade release smoke test panicked with a nil-pointer dereference:

runtime error: invalid memory address or nil pointer dereference
release.getIPerfClientReport(...) test/e2e/release/iperf.go:145
release.(*currentReleaseSmokeTest).verifyIPerfContinuityAfterUpgrade(...) test/e2e/release/current_release_smoke.go:207

Root cause: getIPerfClientReport dereferences iperfServer.Status.MigrationState (a *VirtualMachineMigrationState). verifyIPerfContinuityAfterUpgrade fetched the iperf-server VM immediately, but the post-upgrade workload update live-migrates the VM asynchronously — the migration can start several minutes after the module becomes Ready. At the time of the check MigrationState was still nil, causing the panic. The test never waited for the migration to happen.

This assertion is test-only and was added recently (#2617); it is not a product regression.

Fix (test-only)

• Add waitForIPerfServerMigration in test/e2e/release/iperf.go (styled like waitForIPerfServerToStart) that polls the iperf-server VirtualMachine via f.Clients.VirtClient() until Status.MigrationState != nil, Result == MigrationResultSucceeded, and both StartTimestamp/EndTimestamp are non-zero. Uses framework.MaxTimeout / framework.PollingInterval.
• verifyIPerfContinuityAfterUpgrade now waits for the migration (new By(...) step) and passes the fully-populated VM to getIPerfClientReport.
• Defense-in-depth: getIPerfClientReport gets an explicit Expect(...MigrationState).NotTo(BeNil(), ...) guard so a future regression fails with a clear message instead of a panic.

Verification

• go build ./release/... and go vet ./release/... pass.
• golangci-lint fmt ./release/... (gofumpt/gci/goimports) reports no changes. Note: golangci-lint run currently panics in this environment due to a Go toolchain vs linter version mismatch (Go 1.26 vs linter built with Go 1.25) during package type-checking — unrelated to this change.

section: test
type: fix
summary: "Wait for the iperf server live migration before running the release smoke continuity checks to avoid a nil-pointer panic."
impact_level: low