refactor(shared): split model-availability.ts into model resolution modules

Extract model availability checking pipeline:
- available-models-fetcher.ts: top-level model fetching orchestration
- model-cache-availability.ts, models-json-cache-reader.ts
- provider-models-cache-model-reader.ts: provider cache reading with null guard
- fallback-model-availability.ts, model-name-matcher.ts
- open-code-client-accessors.ts, open-code-client-shapes.ts
- record-type-guard.ts

src/shared/provider-models-cache-model-reader.ts

@@ -0,0 +1,39 @@
+import { readProviderModelsCache } from "./connected-providers-cache"
+import { log } from "./logger"
+
+export function addModelsFromProviderModelsCache(
+	connectedProviders: Set<string>,
+	modelSet: Set<string>,
+): boolean {
+	const providerModelsCache = readProviderModelsCache()
+	if (!providerModelsCache) {
+		return false
+	}
+
+	const providerCount = Object.keys(providerModelsCache.models).length
+	if (providerCount === 0) {
+		log("[fetchAvailableModels] provider-models cache empty, falling back to models.json")
+		return false
+	}
+
+	log("[fetchAvailableModels] using provider-models cache (whitelist-filtered)")
+	const previousSize = modelSet.size
+
+	for (const [providerId, modelIds] of Object.entries(providerModelsCache.models)) {
+		if (!connectedProviders.has(providerId)) continue
+		for (const modelItem of modelIds) {
+			if (!modelItem) continue
+			const modelId = typeof modelItem === "string" ? modelItem : modelItem.id
+			if (modelId) {
+				modelSet.add(`${providerId}/${modelId}`)
+			}
+		}
+	}
+
+	log("[fetchAvailableModels] parsed from provider-models cache", {
+		count: modelSet.size,
+		connectedProviders: Array.from(connectedProviders).slice(0, 5),
+	})
+
+	return modelSet.size > previousSize
+}