Enforce maxKeys limit in mset and fix count tracking for overwrites

[Copilot]

The mset method bypassed maxKeys restrictions, allowing unlimited key insertion. Additionally, both set and mset incorrectly incremented counts when overwriting existing keys, causing count drift from actual stored keys.

Changes:

• maxKeys enforcement in mset: Check limit before each new key insertion, stop processing when reached
• Count tracking fix: Only increment count for new keys, not overwrites. Check key existence before setting to distinguish new vs existing keys
• Performance optimization: Batch existence checks in mset using mget only when maxKeys enabled
• Documentation: Added JSDoc clarifying partial success behavior when limit is reached

Example:

const store = new NodeCacheStore({ maxKeys: 2 });
await store.set("key1", "value1");
await store.set("key2", "value2");

// Previously: Would succeed, bypassing limit
// Now: Stops at limit
await store.mset([
  { key: "key1", value: "updated" },  // ✓ Overwrite succeeds
  { key: "key3", value: "new" },       // ✗ Rejected, limit reached
  { key: "key4", value: "new" },       // ✗ Not processed
]);

Note: The mget implementation still uses sequential get operations rather than true batch retrieval - improving this would require changes to the underlying Keyv store interface.

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