fix(cli): add feature multitenancy keeps the app's own smoke test passing

[omercelikdev]

What broke

With #258–#260 in, the nightly GmGrownRest shape builds and passes spec-lint — and fails its container smoke:

Assert.Equal() Failure: Values differ
Expected: Unauthorized
Actual:   BadRequest

Tenancy is fail-closed: UseGoldpathMultiTenancy refuses a request without X-Goldpath-Tenant with 400, and it runs before auth. The template's smoke sends the header under UseMultiTenancy; goldpath add feature multitenancy never touched the smoke, so a grown app's own smoke test failed. GmGrownRest is the only shape that grows multitenancy through the CLI, and it had never reached its smoke before (restore, then GP1001, stopped it first). The published preview.8 CLI has the same gap.

The fix

Same principle as #260 — the recipe does to template-owned code exactly what the template does:

• the template's line after app.CreateHttpClient("api"); (a parity test reads it from the template),
• using Goldpath; in the smoke and Goldpath.Abstractions in the smoke project,
• a smoke that already sends the header is left alone; an app without the smoke is unaffected; an engine refusal restores both files.

Proof

• Test-first: 2 of 5 new tests red before the fix (the other three are guards). 423/423 CLI tests green after, format clean, all five repo gates green.
• The container smoke cannot run on the maintainer's machine today (Docker is down), so the proof is the nightly dispatched on this branch, which carries all four fixes.

Stacking

On top of #260 (which is on #258 + #259). Merge order: #258, #259, #260, this. None merges until the GM matrix is green with all four (ADR-0008).

Docs: docs/upgrades/0.1.0-preview.9.md (new section with the preview.8 hand fix), docs/guide/cli-reference.md.

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