Fix stats count overcount on key updates in NodeCacheStore

[Copilot]

Both set and mset methods unconditionally incremented the stats count, causing overcount when updating existing keys instead of only tracking unique keys.

Changes

• set method: Check key existence via keyv.has() before incrementing count; only increment for new keys
• mset method: Check each key's existence before incrementing count
• maxKeys enforcement: Allow updates to existing keys when at limit, only block new key additions
• Type consistency: Fix setTtl to accept number | string TTL like other methods

Example

const store = new NodeCacheStore({ maxKeys: 2 });

// Add 2 keys - count is 2
await store.set("key1", "value1");
await store.set("key2", "value2");

// Update existing keys - count remains 2 (previously would become 4)
await store.mset([
  { key: "key1", value: "updated1" },
  { key: "key2", value: "updated2" },
]);

// New key rejected at limit
await store.set("key3", "value3");  // Returns false, count still 2

The has() check adds one async operation per set but is necessary since Keyv's set doesn't distinguish inserts from updates.

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