feat: use cached ALCops analyzers immediately and refresh from NuGet in the background

[Arthurvdv]

Summary

• When a valid cached ALCops analyzer version exists, Ready completes immediately from cache and almcp launches without waiting on NuGet. A background task checks NuGet for a newer version; if found, it is downloaded and cached for the next start.
• On a cold cache (first run) or with a pinned version, the behaviour is unchanged: fetch from NuGet, then cache.
• WorkspaceStartupResolver.GetConfigAsync() is now memoized via Lazy<Task<WorkspaceStartupConfig>>, so the settings.json walk, analyzer resolution and PE metadata reads happen once instead of on every call.

What changed

Cache-first provisioning (AlcopsAnalyzerProvisioner)

• ProvisionCoreAsync checks FindNewestCachedVersion before any network call for Latest/Prerelease modes.
• New FetchAsync / FetchOrFallbackAsync / RefreshCacheAsync split the old monolithic resolve-download block into composable pieces.
• RefreshCacheAsync never throws; it catches cancellation and exceptions, logging a warning on failure.
• ProvisionAsync awaits BackgroundRefresh after setting Ready, so AlcopsAnalyzerProvisionerStartup.StopAsync drains both provisioning and the background refresh.
• FindNewestCachedVersion accepts includePrerelease: Latest skips cached prereleases; Prerelease uses the highest version of any kind. FallbackToCacheOrWarn keeps includePrerelease: true (a cached prerelease with a warning beats nothing when offline).

Carry-overs from review

• AcquireInUseLock documents the FileShare.None cross-process lock assumption (flock on Unix).
• SweepStaleTempDirectories lock probe now catches UnauthorizedAccessException alongside IOException.
• SweepStaleTempDirectories uses BcToolsLocator.SafeEnumerateDirectories (now internal) instead of duplicating the safe-enumerate pattern.
• SweepStaleTempDirectories has three-level error isolation: outer (guards cache root enumeration), per-TFM-folder (guards *.tmp-* enumeration within each TFM folder), and per-directory (guards probe/delete). A failure in one TFM folder never aborts the sweep for any other.
• FallbackToCacheOrWarn has a doc comment explaining the deliberate prerelease inclusion; when the fallback is a prerelease under Latest mode, it logs a specific "no stable version cached" warning instead of the generic "using cached version" line.

Config memoization (WorkspaceStartupResolver)

• _fullConfig is a Lazy<Task<WorkspaceStartupConfig>> initialized in the constructor.
• GetConfigAsync() returns the memoized task; BuildAlMcpArgs uses the completed result or throws if the provisioner is still running.
• With _provisioner == null (the public constructor tests use) the task completes synchronously — existing tests are unaffected.

Behaviour changes

• A freshly published ALCops.Analyzers version is first used on the second start after it ships (visible through the Information log line). First start after publication downloads it in the background.
• Persistent offline runs log one Warning per start (background refresh failure).
• The sync BuildAlMcpArgs now throws InvalidOperationException when a provisioner is attached and not yet ready; no production caller does that (AlMcpProxy.StartCoreAsync uses BuildAlMcpArgsAsync).

Test plan

• dotnet build --configuration Release — 0 warnings, 0 errors
• dotnet test --configuration Release — 159 tests pass (11 new, 1 extended, 1 renamed)
• New: Provision_WarmCache_ReadyFromCache_RefreshDownloadsNewerForNextStart — verifies Ready completes from cache while background refresh is in-flight, then the newer version is cached
• New: Provision_WarmCache_Latest_UsesNewestCachedStable_NotPrerelease — Latest mode skips cached prereleases
• New: Provision_WarmCache_Prerelease_UsesNewestCachedAny — Prerelease mode uses highest cached version of any kind
• New: Provision_WarmCache_RefreshFailure_NeverThrows — background refresh failure is swallowed with a warning
• New: Provision_WarmCache_ShutdownCancelsRefresh — cancelling the token stops the background refresh
• New: Provision_Pinned_Cached_ZeroRequests — pinned version in cache makes zero HTTP requests
• Extended: Provision_ExtractsCorrectTfm_And_WritesManifest — asserts BackgroundRefresh.IsCompleted on cold-cache path
• New: GetConfigAsync_IsMemoized — config resolution is not repeated after settings.json changes
• New: Sweep_EnumerationFailure_DoesNotFailProvisioning — locked stale dir survives sweep; a second TFM folder's stale dir is still cleaned; on Unix, unreadable TFM folder exercises the per-folder catch
• New: Fallback_Latest_OnlyPrereleaseCached_UsesItWithExplicitWarning — prerelease-only cache under Latest logs "no stable version cached"
• Renamed: Provision_Latest_StableAndPrereleaseCached_FastPathUsesStable_NoLastResortWarning (was Fallback_…PrefersStable) — verifies the cache-first fast path, not fallback; asserts zero NuGet requests
• New: Fallback_Prerelease_PicksHighestCachedAcrossStableAndPrerelease — Pinned mode with unresolvable version falls back to highest cached (1.3.0-preview.1 > 1.2.0 in SemVer 2); logs "using cached version"
• All existing provisioner and startup resolver tests remain green
• [AlMcpFact] tests (almcp-backed) remain green

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