test: add LCOWV2 feature flag and v2 LCOW test surface

[shreyanshjain7174]

What

Adds an opt-in -feature LCOWV2 gate across the functional and cri-containerd test suites so the in-progress v2 LCOW controller (internal/controller/vm + internal/builder/vm/lcow) can be exercised end-to-end without disturbing the existing v1 LCOW pipeline.

Why

The v2 controller landed across recent PRs (#2627, #2629, the VM SCSI/VPMem controllers, etc.) but lacked a dedicated functional surface. Existing LCOW tests are tightly coupled to v1 *uvm.OptionsLCOW and call testuvm.CreateLCOW / CreateAndStartLCOWFromOpts — they cannot drive the v2 controller without refactoring every test. A feature flag lets us:

• run the v1 pipeline unchanged on every PR;
• add v2 tests incrementally under TestLCOW_V2_*;
• let CI exercise the v2 path on its own job (continue-on-error: true for now).

Changes

Functional suite (test/functional)

• LCOWV2 implies LCOW in TestMain so featureLCOW-gated tests are reachable; then defaultLCOWOptions calls requireV1Only and every v1 path short-circuits cleanly. Net effect under -feature LCOWV2: only the new TestLCOW_V2_* tests actually run.
• New helpers_v2_test.go and lcow_v2_test.go covering the v2 surface via internal/builder/vm/lcow + the v2 controller in-process.
• Export LCOWBootFilesPath from test/pkg/uvm so v2 tests can resolve boot files without going through v1 *uvm.OptionsLCOW.

cri-containerd suite (test/cri-containerd)

• Mirror the LCOWV2-implies-LCOW pattern in TestMain.
• Thread RuntimeHandler onto the CRI ImageSpec when pulling LCOW images so containerd selects the windows-lcow snapshotter and linux/amd64 platform (the sandbox-platform label alone is not honored by containerd ≥ 2.0).
• Add runhcs-lcow-v2 runtime handler constant and lcow_v2_test.go scaffold.

Flag plumbing (test/pkg/flag)

• Add IncludesExplicit and Include on IncludeExcludeStringSet so test TestMain hooks can implement feature implications safely after flag.Parse without breaking default-when-unset semantics.

CI (.github/workflows/ci.yml)

• New Build and run functional testing binary (LCOWV2) step that invokes functional.test.exe -feature=LCOWV2 -exclude=LCOWIntegrity. Marked continue-on-error: true while the v2 surface is being grown so a v2-specific failure does not block the v1 pipeline.
• Build and upload containerd-shim-lcow-v2.exe as a test artifact.

Out of scope (follow-up)

• CRI v2 testing (Test_V2_LCOW_* in test/cri-containerd) requires a CI step that starts containerd with snapshotter = "windows-lcow" set on both the runhcs-lcow AND runhcs-lcow-v2 runtime blocks. That step is deferred to a follow-up PR alongside the integration-tests v2 setup.
• Growing the TestLCOW_V2_* surface beyond the initial scaffold.

Validation

• Local: 10 v2 functional tests pass against the v2 controller via functional.test.exe -feature=LCOWV2 on a Windows host.
• v1 pipeline: unchanged — existing -feature LCOW invocations route to the same code paths as before.
• CRI v2 tests: do not pass locally against vanilla containerd because the upstream CRI plugin hardcodes platforms.DefaultSpec() in image_pull.go and ignores the runtime_platforms mapping; gating those tests on CI is intentionally deferred.

DCO

Signed-off-by in commit.