[feat] Rewrite stub + comprehensive mirror pull tests

[ldmonster]

Overview

This PR replaces the monolithic, brittle pkg/stub/registry_client.go (932 lines) with a clean, composable in-memory stub library split into focused files. It then uses the new stub to write 168 passing tests across the entire mirror pull pipeline — platform, installer, security, modules, and the top-level PullService.

---

Why

The previous stub was a single 932-line file that embedded hard-coded switch statements to guess version.json content from tag names, re-implemented raw gzip/tar construction manually, and had no way to register custom repos or images. Adding new tests required patching the stub itself. Any image the stub hadn't anticipated returned wrong data silently.

---

What changed

stub — rewritten from scratch

File	Purpose
registry_client.go	deleted
registry.go	In-memory registry: per-repo image store, MustAddImage, tag listing
client.go	Client implementing localreg.Client via WithSegment path routing
image.go	ImageBuilder fluent API: WithFile, WithLabel, MustBuild
deckhouse_stub.go	NewRegistryClientStub() — Deckhouse-shaped default fixture
client_test.go / registry_test.go / image_test.go / deckhouse_stub_test.go	Full coverage of all four new files

Key design properties:

• Composable: NewRegistry(host) + reg.MustAddImage(repo, tag, img) — tests control exactly what's in the registry.
• No inference: images contain what you put in them; no tag-name guessing.
• Path-routed: client.WithSegment("release-channel") builds a scoped client for the release-channel repo — identical interface to the real registry client.
• Push support: PushImage auto-creates repos, making push-path tests possible.

service.go

NewService now skips WithSegment(edition) when edition == pkg.NoEdition. This prevents double-scoping when the stub's host URL already includes the edition path segment.

platform_dryrun.go

Extracted the dry-run execution path for PullPlatform into a separate file to keep platform.go focused on the real pull logic.

pull.go

When STUB_REGISTRY_CLIENT=true the stub client is substituted and edition is reset to pkg.NoEdition, preventing the double-scope bug (…/fe/fe).

mirror-pull-dry-run.md

New architecture document describing the DryRun execution path, what each service does/skips, and the stub fixture design.