add pipeline to test next gen vm series

[lilypan26]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

[github-actions]

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Current Title: add pipeline to test next gen vm series

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[Copilot]

Pull request overview

Adds a new Azure DevOps pipeline definition to run targeted AgentBaker E2E scenarios intended to cover “next gen” Linux VM series.

Changes:

• Introduced .pipelines/e2e-next-gen.yaml with variables/tags to run VM-series-coverage E2E scenarios.
• Wired the pipeline to run on main and on PRs affecting E2E- and provisioning-related paths.

[Copilot]

DEFAULT_VM_SIZE is not a recognized env var in the e2e test harness. The e2e config reads DEFAULT_VM_SKU (see e2e/config/config.go), so this pipeline will keep using the default VM size and not exercise the intended next-gen SKU. Rename this variable to DEFAULT_VM_SKU (and use the expected SKU string format) so the new pipeline actually runs on the desired VM series.

Suggested change

	DEFAULT_VM_SIZE: "standard_d2ds_v7"
	DEFAULT_VM_SKU: "Standard_D2ds_v7"