[fix] respect content-range header in resumable upload

[JornEngelbart]

Fix Content-Range header handling in resumable uploads

Problem

The resumable upload implementation was not correctly handling the Content-Range header when uploading chunks. The code was simply appending all chunks sequentially (obj.Content = append(obj.Content, content...)), which caused incorrect behavior when:

1. Overlapping ranges: Clients may re-upload the same byte range (e.g., for retry scenarios)
2. Out-of-order chunks: Though rare, chunks might arrive out of sequence
3. Non-sequential uploads: Some clients upload chunks that don't start at position 0

This resulted in:

• Content being placed at incorrect byte positions
• Duplicate content when ranges overlapped
• Incorrect file sizes and corruption

Solution

The fix properly parses and respects the Content-Range header to place content at the exact byte positions specified:

Key Changes

1. Parse Content-Range before appending: The code now parses the Content-Range header first to determine where content should be placed

2. Place content at specified positions: Instead of appending, content is copied to the exact byte range specified:

   // For known ranges (e.g., "bytes 0-262143/377464")
   copy(obj.Content[contentStart:contentStart+len(content)], content)

3. Handle overlapping ranges: When the same range is uploaded multiple times, the content correctly overwrites the previous content at those positions

4. Extend buffer as needed: The buffer is automatically extended with zero-filled bytes when a range requires a larger buffer size

5. Validate content length: Added validation to ensure the actual content length matches the Content-Range header specification

Example Scenario

Previously, if a client sent:

• Request 1: Content-Range: bytes 0-262143/377464 (262144 bytes)
• Request 2: Content-Range: bytes 1-262144/377464 (262144 bytes, overlapping)
• Request 3: Content-Range: bytes 1-377463/377464 (377463 bytes, overlapping)

The old code would append all chunks sequentially, resulting in an incorrect file.

The new code correctly:

• Places first chunk at positions 0-262143
• Places second chunk at positions 1-262144 (overwriting positions 1-262143 and adding new byte at 262144)
• Places third chunk at positions 1-377463 (overwriting and extending as needed)
• Final result: Correct file with all bytes in the right positions

Testing

Added comprehensive test coverage in TestResumableUploadWithContentRange that verifies:

• Overlapping range uploads (same range uploaded twice)
• Sequential non-overlapping range uploads
• Proper content placement at specified byte positions
• Checksum validation of final content

Impact

This fix ensures that fake-gcs-server correctly handles resumable uploads with Content-Range headers, making it compatible with clients that:

• Retry uploads with overlapping ranges
• Upload chunks in non-sequential order
• Use the Content-Range header to specify exact byte positions

The fix is backward compatible and maintains the existing behavior for uploads without Content-Range headers (falls back to append behavior).