fix(usenet): prevent memory leak by closing all segments in Clear()

[rolandb5]

Summary

• Fix memory leak in segmentRange.Clear() that caused unbounded memory growth
• The method was returning early on first error, leaving remaining segments unclosed
• Now continues closing all segments and returns joined errors

Problem

When downloads stalled or failed, Clear() would exit on the first segment close error, leaving other segments open. Each unclosed segment holds buffers, causing memory to grow unbounded (observed 4+ GB growth in production).

Solution

• Modified Clear() to iterate through all segments regardless of individual close errors
• Collect all errors and return them joined using errors.Join()
• Added comprehensive test coverage for edge cases

Testing

• Added unit tests covering:
  • Empty range
  • Single segment (success/error)
  • Multiple segments (all success, all error, mixed)
  • Nil segment handling
• Validated in production: memory stable at 20-50 MB over 9 hours (previously grew to 2.5 GB)

Impact

Critical fix - Without this, the container would eventually OOM during active streaming of large files.

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