Improve test coverage for edge cases and cache failures

While reviewing the test suite, I noticed some gaps in coverage, particularly around error handling and edge cases that could occur in production.

### Issues
1. Missing tests for concurrent cache access patterns
2. No tests for cache connection timeouts or failures
3. Limited coverage of edge cases with unusual input types

### Proposed Tests to Add
1. **Concurrent operations test**:
```typescript
test('should handle concurrent operations correctly', async () => {
const testBatchLoadFn = jest.fn(async (keys) => {
// Add a slight delay to simulate db access
await new Promise(resolve => setTimeout(resolve, 50));
return keys.map(key => `Value for ${key}`);
});

const loader = new KeyvDataLoader({
batchLoadFn: testBatchLoadFn,
ttl: 1000,
});

// Run many loads concurrently
const promises = Array.from({ length: 100 }, (_, i) =>
loader.load(`key-${i % 10}`) // Only 10 unique keys to test deduplication
);

const results = await Promise.all(promises);

// Verify results are correct
expect(results.length).toBe(100);

// Verify batch function was called efficiently
// Should be called at most 10 times (once per unique key)
expect(testBatchLoadFn.mock.calls.length).toBeLessThanOrEqual(10);
});
```

2. **Cache failure test**:
```typescript
test('should handle cache failures gracefully', async () => {
// Create a mock Keyv implementation that throws on operations
const failingCache = {
get: jest.fn().mockRejectedValue(new Error('Cache failure')),
set: jest.fn().mockRejectedValue(new Error('Cache failure')),
delete: jest.fn().mockRejectedValue(new Error('Cache failure')),
clear: jest.fn().mockRejectedValue(new Error('Cache failure')),
};

// Override the Keyv constructor to return our failing implementation
jest.mock('keyv', () => {
return jest.fn().mockImplementation(() => failingCache);
});

const testBatchLoadFn = jest.fn(async (keys) => {
return keys.map(key => `Value for ${key}`);
});

const loader = new KeyvDataLoader({
batchLoadFn: testBatchLoadFn,
ttl: 1000,
});

// Should still work even with cache failures
const result = await loader.load('test-key');
expect(result).toBe('Value for test-key');

// Batch function should be called because cache failed
expect(testBatchLoadFn).toHaveBeenCalled();
});
```

3. **Edge case tests for unusual inputs**:
```typescript
test('should handle special characters in cache keys', async () => {
const loader = new KeyvDataLoader({
batchLoadFn: async (keys) => keys.map(key => `Value for ${key}`),
ttl: 1000,
});

const specialKeys = [
'key with spaces',
'key:with:colons',
'key/with/slashes',
'key+with+plus',
'key&with&ersand',
'key#with#hash',
'key?with?question',
JSON.stringify({complex: 'object', with: {nested: 'values'}}),
];

for (const key of specialKeys) {
const result = await loader.load(key);
expect(result).toBe(`Value for ${key}`);

// Load again to verify caching works
const cachedResult = await loader.load(key);
expect(cachedResult).toBe(`Value for ${key}`);
}
});
```

These additional tests would help ensure the library works correctly in more real-world scenarios and improves the overall reliability of the codebase.