Standardize go:generate for controller mocks + CI drift validation

[shreyanshjain7174]

Background

Per @rawahars's review on #2647: "Each controller has mocks needs to have the generate directive. The go:generate directive should reside in the test file."

This issue tracks the rollout for all controllers + a CI gate.

Goals

1. Standardise mockgen invocation behind a wrapper script so individual //go:generate lines stay short and consistent.
2. Add //go:generate directive in a TEST file (not source file) for each controller, using the wrapper.
3. Add CI validation that runs go generate ./... and fails if there is any diff against the committed mock files.

Prior art — AWS amazon-ecs-agent

AWS uses a wrapper script: https://github.com/aws/amazon-ecs-agent/blob/master/scripts/mockgen.sh

Key design choices worth borrowing:

• Single short //go:generate scripts/mockgen.sh <pkg> <source> <outdir> directive in source files
• Wrapper runs mockgen | goimports to normalise imports
• Wrapper prepends a standard licence/header banner so all generated files are byte-identical regardless of mockgen version drift

Suggested directive shape (hcsshim)

For platform-divergent guest interfaces (e.g. network.guestNetwork), the wrapper would resolve -build_constraint automatically based on filename suffix (_lcow.go -> windows&&lcow), so contributors don't have to spell it out.

Single short directive in a test file:

//go:generate scripts/mockgen.ps1 mocks types.go vmLifetime,guestManager

Wrapper handles -build_flags, -package, -destination, gofmt, banner.

Affected packages (current scope)

• internal/controller/device/scsi/
• internal/controller/device/plan9/
• internal/controller/device/vpci/
• internal/controller/network/
• internal/controller/vm/
• internal/windows/ (mock at internal/windows/mock/)
• internal/gcs/ (GuestDefinedCapabilities mock at internal/gcs/mock/)
• cmd/containerd-shim-lcow-v2/service/

Suggested CI step

Add a job (or extend an existing lint job) in .github/workflows/ci.yml:

- name: Verify generated mocks are up to date
  run: |
    go generate ./...
    git diff --exit-code -- '*/mocks/*.go' '*/mock/*.go'

Run on windows-latest so build tags resolve correctly.

Acceptance criteria

• scripts/mockgen.ps1 (or .sh) wrapper exists and is documented
• All listed controllers have a //go:generate directive in a test file using the wrapper
• go generate ./... produces no diff against committed mocks on a fresh clone
• CI fails if a contributor changes an interface without regenerating
• Documentation (CONTRIBUTING.md or similar) explains: "After modifying an interface, run go generate ./... and commit the regenerated mocks"

Notes

Discussed in #2647 (rawahars review on internal/controller/network/network_lcow.go).