fix(storage-s3): validate Content-Length before appending header (#13472)

## Description
Fixes "Parse Error: Invalid character in Content-Length" errors that
occur when S3-compatible storage providers (like MinIO) return undefined
or invalid ContentLength values.

## Changes
- Added validation before appending Content-Length header in
`staticHandler.ts`
- Only appends Content-Length when value is present and numeric
- Prevents HTTP specification violations from undefined/invalid values

## Code Changes
```typescript
const contentLength = String(object.ContentLength);
if (contentLength && !isNaN(Number(contentLength))) {
  headers.append('Content-Length', contentLength);
}
```

## Issue
- Resolves MinIO compatibility issues where undefined ContentLength
causes client parse errors
- Maintains backward compatibility when ContentLength is valid

## Testing
- [x] Tested with MinIO provider returning undefined ContentLength
- [x] Verified valid Content-Length values are still properly set
- [x] Confirmed no regression in existing S3 functionality

### Type of Change
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to not work as expected)

### Checklist
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings
- [x] Any dependent changes have been merged and published
```