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/models-json-cache-reader.ts

@@ -0,0 +1,52 @@
+import { existsSync, readFileSync } from "fs"
+import { join } from "path"
+import { getOpenCodeCacheDir } from "./data-path"
+import { log } from "./logger"
+import { isRecord } from "./record-type-guard"
+
+export function addModelsFromModelsJsonCache(
+	connectedProviders: Set<string>,
+	modelSet: Set<string>,
+): boolean {
+	const cacheFile = join(getOpenCodeCacheDir(), "models.json")
+	if (!existsSync(cacheFile)) {
+		log("[fetchAvailableModels] models.json cache file not found, falling back to client")
+		return false
+	}
+
+	try {
+		const content = readFileSync(cacheFile, "utf-8")
+		const data: unknown = JSON.parse(content)
+		if (!isRecord(data)) {
+			return false
+		}
+
+		const providerIds = Object.keys(data)
+		log("[fetchAvailableModels] providers found in models.json", {
+			count: providerIds.length,
+			providers: providerIds.slice(0, 10),
+		})
+
+		const previousSize = modelSet.size
+		for (const providerId of providerIds) {
+			if (!connectedProviders.has(providerId)) continue
+			const providerValue = data[providerId]
+			if (!isRecord(providerValue)) continue
+			const modelsValue = providerValue["models"]
+			if (!isRecord(modelsValue)) continue
+			for (const modelKey of Object.keys(modelsValue)) {
+				modelSet.add(`${providerId}/${modelKey}`)
+			}
+		}
+
+		log("[fetchAvailableModels] parsed models from models.json (NO whitelist filtering)", {
+			count: modelSet.size,
+			connectedProviders: Array.from(connectedProviders).slice(0, 5),
+		})
+
+		return modelSet.size > previousSize
+	} catch (err) {
+		log("[fetchAvailableModels] error", { error: String(err) })
+		return false
+	}
+}