[v0.8.3] Offline-registry check fix

[LittleCoinCoin]

Summary

• Read-only commands (e.g. hatch mcp list servers) now keep working when the registry is unreachable and no usable cache exists, instead of crashing with a traceback
• RegistryRetriever.get_registry() returns a REGISTRY_UNAVAILABLE sentinel when offline with no cache, rather than raising
• Remote installs now fail with a clear "registry is unavailable" message instead of an opaque error; local-package installs are unaffected
• Adds 2 offline regression tests covering the no-cache and stale-cache paths (7/7 offline regression tests pass)

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Bug Fixes

Registry retrieval crashed when offline with no cache (hatch/registry_retriever.py)

Root cause: When the remote registry could not be fetched and no local cache file existed, get_registry() propagated the fetch exception. Every command that touches the registry on startup — including purely read-only ones — failed with a traceback the moment the machine was offline.

Example: hatch mcp list servers on a laptop with no network and a cold cache → unhandled exception instead of an (empty) listing.

Solution:

• Introduced a module-level REGISTRY_UNAVAILABLE sentinel ({"status": "unavailable", "repositories": []}).
• Wrapped the remote fetch so that on failure it falls back to the cached registry when one exists (logging a "may be stale" warning), and only returns the sentinel when there is no cache at all.
• The sentinel return path is an early return that bypasses _write_local_cache, the in-memory cache update, and _save_last_fetch_time, so the placeholder is never persisted to disk.
• Downgraded the "registry not found for today/yesterday" log from error to warning, with a message that names offline/unpublished-release as the likely cause.

Test coverage:

• test_offline_no_cache_returns_unavailable_sentinel — offline with no cache returns the sentinel and asserts the cache file is never written.
• test_offline_stale_cache_returns_stale_data_not_sentinel — offline with a stale cache returns the real (stale) data, not the sentinel.

Remote install raised an opaque error against an unavailable registry (hatch/installers/dependency_installation_orchestrator.py)

Root cause: With the registry now able to be unavailable, the remote-install path would proceed to query a registry with no repositories and surface a confusing failure.

Solution:

• Guard the remote-package branch: when registry_data.get("status") == "unavailable", raise a DependencyInstallationError with an actionable message — "registry is unavailable. Connect to the internet and retry, or use a local package path."
• Local-package installs do not consult the registry and are therefore unaffected.

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Developer Experience

Offline regression coverage

The two new tests in tests/test_registry_retriever.py lock in the offline contract: the sentinel is returned (never persisted) when there is no cache, and a stale cache is preferred over the sentinel. Combined with the existing offline checks, 7/7 offline regression tests pass.