FixedLengthStream API Test Cases

Use query parameter ?test=<test number> to select a test to run
Example: /fixed-length?test=2

Available tests:
1. Normal case - Write exactly 1KB of data
2. Write too little data - Only write 5 bytes, should cause an error
3. Write too much data - Try to write 15 bytes, should cause an error
4. No close after write - Write data but don't call close()
5. Using TextEncoder - Ensure correct byte encoding
6. Write non-ASCII characters - Test multi-byte character handling
7. Large file test - Create a larger FixedLengthStream
8. Zero length test - Create a FixedLengthStream with length 0
9. Error handling test - Deliberately trigger and handle errors
10. Multiple writes test - Multiple small batch writes