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/fallback-model-availability.ts

@@ -0,0 +1,67 @@
+import { readConnectedProvidersCache } from "./connected-providers-cache"
+import { log } from "./logger"
+import { fuzzyMatchModel } from "./model-name-matcher"
+
+export function isAnyFallbackModelAvailable(
+	fallbackChain: Array<{ providers: string[]; model: string }>,
+	availableModels: Set<string>,
+): boolean {
+	if (availableModels.size > 0) {
+		for (const entry of fallbackChain) {
+			const hasAvailableProvider = entry.providers.some((provider) => {
+				return fuzzyMatchModel(entry.model, availableModels, [provider]) !== null
+			})
+			if (hasAvailableProvider) {
+				return true
+			}
+		}
+	}
+
+	const connectedProviders = readConnectedProvidersCache()
+	if (connectedProviders) {
+		const connectedSet = new Set(connectedProviders)
+		for (const entry of fallbackChain) {
+			if (entry.providers.some((p) => connectedSet.has(p))) {
+				log(
+					"[isAnyFallbackModelAvailable] model not in available set, but provider is connected",
+					{ model: entry.model, availableCount: availableModels.size },
+				)
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
+export function isAnyProviderConnected(
+	providers: string[],
+	availableModels: Set<string>,
+): boolean {
+	if (availableModels.size > 0) {
+		const providerSet = new Set(providers)
+		for (const model of availableModels) {
+			const [provider] = model.split("/")
+			if (providerSet.has(provider)) {
+				log("[isAnyProviderConnected] found model from required provider", {
+					provider,
+					model,
+				})
+				return true
+			}
+		}
+	}
+
+	const connectedProviders = readConnectedProvidersCache()
+	if (connectedProviders) {
+		const connectedSet = new Set(connectedProviders)
+		for (const provider of providers) {
+			if (connectedSet.has(provider)) {
+				log("[isAnyProviderConnected] provider connected via cache", { provider })
+				return true
+			}
+		}
+	}
+
+	return false
+}