[TFgen] Phase 1 Setup CLI

[j10czar]

APIR-2491

What this does

Phase 1 scaffolding for the new generator (tfgen): build, CI, and test-fixture
plumbing so later phases have a consistent place to land. No generator logic
yet. Targets the generator-v2 branch, which already has the stubs and deps.

Changes

• CI workflow (.github/workflows/tfgen.yml): runs go build and go vet
  against the generator on any PR that touches .generator-v2/internal/**,
  .generator-v2/cmd/tfgen/**, or the workflow file. Build and vet only for
  now. Unit tests, e2e, coverage, and the generate --check / verify --strict
  gates come in later phases.

• Make targets: make tfgen-build builds the binary to bin/tfgen,
  make tfgen-test runs go test ./internal/... ./cmd/tfgen/.... Both run
  inside the .generator-v2 module and honor a GO= override.

• Fixture catalogue (.generator-v2/internal/testdata/fixtures/README.md):
  documents the per-fixture layout (input openapi.yaml, committed golden
  out/, optional README) so e2e fixtures get added consistently. No fixtures
  included here.

Testing

• make tfgen-build produces bin/tfgen.
• make tfgen-test exits 0 (no test files yet).
• Workflow runs build + vet clean.

[Supam]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [TFgen] Phase 1 Setup CLI #3842 👈 (View in Graphite)
• [TFgen] Adding a new terraform generator #3840 : 8 other dependent PRs (#3849 , #3850 , #3851 and 5 others)
• master

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