fix(model-resolver): skip fallback chain when no cache exists

When no provider cache exists, skip the fallback chain entirely and let
OpenCode use Provider.defaultModel() as the final fallback. This prevents
incorrect model selection when the plugin loads before providers connect.

- Remove forced first-entry fallback when no cache
- Add log messages for cache miss scenarios
- Update tests for new behavior

src/shared/model-resolver.ts

@@ -58,25 +58,26 @@ export function resolveModelWithFallback(
 			const connectedProviders = readConnectedProvidersCache()
 			const connectedSet = connectedProviders ? new Set(connectedProviders) : null
 
-			for (const entry of fallbackChain) {
-				for (const provider of entry.providers) {
-					if (connectedSet === null || connectedSet.has(provider)) {
-						const model = `${provider}/${entry.model}`
-						log("Model resolved via fallback chain (no model cache, using connected provider)", { 
-							provider, 
-							model: entry.model, 
-							variant: entry.variant,
-							hasConnectedCache: connectedSet !== null
-						})
-						return { model, source: "provider-fallback", variant: entry.variant }
+			// When no cache exists at all, skip fallback chain and fall through to system default
+			// This allows OpenCode to use Provider.defaultModel() as the final fallback
+			if (connectedSet === null) {
+				log("No cache available, skipping fallback chain to use system default")
+			} else {
+				for (const entry of fallbackChain) {
+					for (const provider of entry.providers) {
+						if (connectedSet.has(provider)) {
+							const model = `${provider}/${entry.model}`
+							log("Model resolved via fallback chain (no model cache, using connected provider)", { 
+								provider, 
+								model: entry.model, 
+								variant: entry.variant,
+							})
+							return { model, source: "provider-fallback", variant: entry.variant }
+						}
 					}
 				}
+				log("No matching provider in connected cache, falling through to system default")
 			}
-			const firstEntry = fallbackChain[0]
-			const firstProvider = firstEntry.providers[0]
-			const model = `${firstProvider}/${firstEntry.model}`
-			log("Model resolved via fallback chain (no cache at all, using first entry)", { provider: firstProvider, model: firstEntry.model, variant: firstEntry.variant })
-			return { model, source: "provider-fallback", variant: firstEntry.variant }
 		}
 
 		for (const entry of fallbackChain) {