Refactor: Simplify bucket policy error handling

Summary

This PR simplifies error handling in policy_sys.rs by directly using StorageError::ConfigNotFound instead of converting through BucketMetadataError. This change reduces unnecessary type conversions and aligns with the unified error handling approach.

Changes

• Removed BucketMetadataError import and type conversion
• Changed error check to directly compare with StorageError::ConfigNotFound
• Updated log level from warn to info for config not found errors
• Simplified error handling logic in is_allowed method

Code Changes

Before

use super::{error::BucketMetadataError, metadata_sys::get_bucket_metadata_sys};
// ...
Err(err) => {
    let berr: BucketMetadataError = err.into();
    if berr != BucketMetadataError::BucketPolicyNotFound {
        warn!("config get err {:?}", berr);
    }
}

After

use crate::error::{Result, StorageError};
// ...
Err(err) => {
    if err != StorageError::ConfigNotFound {
        info!("config get err {:?}", err);
    }
}

Benefits

1. Simplified code: Removes unnecessary type conversion step
2. Consistent error handling: Uses unified StorageError type directly
3. Reduced complexity: Fewer imports and type conversions
4. Better log level: Changed from warn to info for expected "not found" scenarios

Related Issues

Fixes #823