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-shapes.ts

@@ -0,0 +1,7 @@
+export type ProviderListResponse = { data?: { connected?: string[] } }
+export type ModelListResponse = {
+	data?: Array<{ id?: string; provider?: string }>
+}
+
+export type ProviderListFunction = () => Promise<ProviderListResponse>
+export type ModelListFunction = () => Promise<ModelListResponse>