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/open-code-client-accessors.ts

@@ -0,0 +1,20 @@
+import type { ModelListFunction, ProviderListFunction } from "./open-code-client-shapes"
+import { isRecord } from "./record-type-guard"
+
+export function getProviderListFunction(client: unknown): ProviderListFunction | null {
+	if (!isRecord(client)) return null
+	const provider = client["provider"]
+	if (!isRecord(provider)) return null
+	const list = provider["list"]
+	if (typeof list !== "function") return null
+	return list as ProviderListFunction
+}
+
+export function getModelListFunction(client: unknown): ModelListFunction | null {
+	if (!isRecord(client)) return null
+	const model = client["model"]
+	if (!isRecord(model)) return null
+	const list = model["list"]
+	if (typeof list !== "function") return null
+	return list as ModelListFunction
+}