docs: Fix outdated examples and update documentation for boto3-compatible responses

Updated all documentation to reflect the boto3-compatible dict responses:
- Fixed pagination examples in README.md to use dict access
- Updated docs/sdk/api.md with correct list_objects() signature and examples
- Added return type documentation for list_objects()
- Updated CHANGELOG.md with breaking changes and migration info

All examples now use:
- response['Contents'] instead of response.contents
- response.get('IsTruncated') instead of response.is_truncated
- response.get('NextContinuationToken') for pagination

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -207,14 +207,18 @@ with open('downloaded.zip', 'wb') as f:
 
 # Smart list_objects with optimized performance
 response = client.list_objects(Bucket='releases', Prefix='v2.0.0/')
+for obj in response['Contents']:
+    print(f"{obj['Key']}: {obj['Size']} bytes")
 
 # Paginated listing for large buckets
 response = client.list_objects(Bucket='releases', MaxKeys=100)
-while response.is_truncated:
+while response.get('IsTruncated'):
+    for obj in response['Contents']:
+        print(obj['Key'])
     response = client.list_objects(
         Bucket='releases',
         MaxKeys=100,
-        ContinuationToken=response.next_continuation_token
+        ContinuationToken=response.get('NextContinuationToken')
     )
 
 # Delete and inspect objects