Fix stringio content length

[killerdevildog]

fix(utils): correct Content-Length for io.StringIO bodies (#6917)

• Add special handling for io.StringIO in super_len() function
• Calculate byte length by encoding StringIO content as UTF-8
• Preserve stream position to avoid surprising users
• Fixes incorrect Content-Length header when StringIO contains multi-byte UTF-8 characters
• Test multi-byte UTF-8 characters (💩, 🚀, 🎉)
• Test position preservation after super_len call
• Test partial read scenarios
• Test PreparedRequest Content-Length header
• Test consistency with str and bytes behavior
• All tests verify correct byte-length calculation"