Fix stats count tracking to only increment for new keys, not updates

[Copilot]

Stats count was being incremented unconditionally after every set() operation, causing overcounting when updating existing keys.

Changes

• set() method: Check if key exists via this._keyv.has() before incrementing count. Only new keys increment the counter.
• mset() method: Apply same logic for batch operations - each item checks existence before count increment.
• maxKeys enforcement: Updated to only reject new keys when at capacity. Updates to existing keys are allowed regardless of limit.

Example

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

await store.set("key1", "value1");  // count = 1 ✓
await store.set("key2", "value2");  // count = 2 ✓
await store.set("key1", "updated");  // count = 2 (not 3) ✓
await store.set("key3", "value3");  // rejected (at limit) ✓

Tests added to verify count behavior for both single and batch operations.

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