[cccplex]

Problem

Failed model discovery is cached as an empty list for five minutes,
hiding available models and replacing previously successful results.

Fix

• Preserve cached models and expiration on failure.
• Retry discovery on the next call; no added retry loop.
• Still cache genuinely successful empty results.

This fixes failure caching, not the underlying invalid API response.

Tests

11 regression tests added; eight fail without the fix.
All 102 tests, StyLua, and whitespace checks pass.
Auto chat manually verified, including after restarting Neovim.

Related: #1343 also changes discovery error handling; this approach preserves
cached models and allows other providers to remain usable.

[pullfrog]

✅ No new issues found.

Reviewed changes

First review of this model-cache recovery fix (1 commit, 2 files).

• get_cached preserves stale data on failure (lua/CopilotChat/client.lua:83-91) — a nil from the filler now returns any previously cached value and leaves _expires_at untouched, so the entry stays expired and is refetched on the next call instead of caching an empty result for the full 5-minute TTL. Because the inner cache is a plain table (not the defaulttable wrapper), cache[key] or {} does not auto-vivify and _expires_at is only ever written alongside cache[key], so the _expires_at > now comparison can't be reached with a nil/table value.
• Client:models reports failure as nil (lua/CopilotChat/client.lua:243-255) — the auth-failure and get_models-failure branches now return nil, and a nil provider result is passed through unchanged, engaging the recovery path. Genuinely successful empty lists are still cached.
• New regression suite (tests/client_spec.lua) — 243 lines covering first-fetch caching, retry after auth/model-fetch failure, the expiry boundary, stale preservation then recovery, preservation when auth refresh fails, persistent-failure retry without an internal loop, genuine empty-list caching, nil provider result not cached, independent per-provider caching, and an end-to-end auto recovery after an invalid JSON response.

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