Add safety limit to Cloudflare list_keys pagination #202

Description

The Cloudflare CloudflareKvStore::list_keys implementation paginates through all keys in a KV namespace and accumulates them into a single Vec<String>. On namespaces with a large number of keys, this unbounded accumulation could cause an out-of-memory condition.

What needs to be done

Add a safety limit to list_keys pagination in crates/edgezero-adapter-cloudflare/src/key_value_store.rs:

• Cap the total number of keys returned (e.g., 10,000) or the total accumulated size
• Return a clear error when the limit is hit, rather than silently truncating
• Consider whether list_keys should accept an optional limit parameter at the KvStore trait level

Done when

• CloudflareKvStore::list_keys has a configurable or hardcoded safety limit
• Exceeding the limit returns a meaningful KvError
• Behavior is documented

Affected area

Adapter -- Cloudflare

Context

Identified during PR #165 (KV store feature) review. The current implementation works correctly for small namespaces but poses an OOM risk in production with large key sets.