chore: move apiversion package into tools/foas

[yelizhenden-mdb]

What this introduces

Branch 2 of 4 in splitting tools/cli into a library (tools/foas) + binary (tools/cli). Builds on #1308 (now merged). This is the pilot move: it relocates one package — apiversion — to prove out the cross-module wiring before moving the rest.

Changes

• Moved tools/cli/internal/apiversion/ → tools/foas/apiversion/ (4 files: version.go, stabilitylevel.go, + tests). Package name unchanged (package apiversion).
• Rewrote imports in all 20 cli callers from …/tools/cli/internal/apiversion to …/tools/foas/apiversion.
• Deleted tools/cli/pkg/apiversion/ — a thin re-export wrapper with zero importers (dead code; the whole point of tools/foas is to be the public surface).
• Wired the module dependency in tools/cli/go.mod:

  require github.com/mongodb/openapi/tools/foas v0.0.0
  replace github.com/mongodb/openapi/tools/foas => ../foas
  

  go.work resolves foas locally during dev/CI; the replace covers GOWORK=off (GoReleaser).
• go mod tidy surfaced that apiversion (and its tests) pull in kin-openapi and testify, so tools/foas now declares those directly — hence the new tools/foas/go.sum and regenerated tools/go.work.sum.

Verification

This PR only moves the apiversion package (no logic change; package name and behavior unchanged; the expected-output specs committed in the repo are untouched). The foascli binary is consumed by several release/generation workflows (release-cli, release-spec, release-changelog, generate-openapi, changelog-report, required-spec-validations, api-versions-reminder), so behavioral parity is verified end-to-end:

CI (Linux) — all green on this PR:

Check	Result
e2e-tests (builds foascli, runs commands, compares output against the expected specs committed in the repo)	✅ pass
build (foascli + foas-lib)	✅ pass
lint (both modules)	✅ pass
unit-tests (ubuntu + macos, both modules)	✅ pass

Local (macOS) — second, independent signal:

• apiversion-dependent e2e tests (split, changelog, versions) → ok …/test/e2e/cli 157.001s (exit 0)

Because the e2e suite drives the real binary and compares its output against the expected specs committed in the repo (via an oasdiff structural comparison that must come back empty), passing e2e proves the CLI produces the same output as before — the consuming workflows are unaffected.

Next branches

• Branch 3 — move breakingchanges, changelog, openapi, pointer; delete tools/cli/pkg/ entirely
• Branch 4 — CI & release wiring (rename code-health-foascli.yml → code-health-foas-cli.yml with a tools/foas/** trigger; add release-foas-lib.yml)

[andmatei]

LGTM