fix(kv,cache): merge default binding when driver explicitly set

[onmax]

Fixes #766

Problem

When explicitly setting driver: 'cloudflare-kv-binding' in hub.kv/cache config, resolveKVConfig() and resolveCacheConfig() return early without merging the default binding. This causes unstorage to fall back to 'STORAGE' binding which doesn't exist.

Solution

Merge default binding ('KV' for kv, 'CACHE' for cache) when driver is explicitly set to cloudflare-kv-binding.

Reproduce bug

git clone --depth 1 --filter=blob:none --sparse https://github.com/onmax/repros.git
cd repros && git sparse-checkout set nuxthub-766
cd nuxthub-766 && pnpm i && pnpm build && cat .nuxt/hub/kv.mjs

Shows driver({}) - empty options.

Verify fix

git sparse-checkout add nuxthub-766-fixed
cd ../nuxthub-766-fixed && pnpm i && pnpm build && cat .nuxt/hub/kv.mjs

Shows driver({"binding":"KV"}).

Test?

Let me know if we should include tests to avoid future regressions

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