Fix incorrect Content-Length for StringIO with multi-byte characters

[veeceey]

Summary

Fixes #6917.

super_len() uses seek/tell to measure the length of file-like objects such as StringIO and BytesIO. However, StringIO.tell() returns the character position, not the byte offset. For strings containing multi-byte UTF-8 characters (e.g. emoji), this produces an incorrect Content-Length header that violates RFC 9110 section 8.6.

For example, io.StringIO("\U0001F4A9") (a single emoji) previously returned a length of 1 (character count) instead of 4 (UTF-8 byte count), causing the server to receive a Content-Length: 1 header while 4 bytes are actually sent.

This is the same class of bug that was fixed for plain str bodies in #6586 -- str is encoded to UTF-8 before measuring, but StringIO was not. This PR makes StringIO handling consistent with str by reading the remaining text, encoding it to UTF-8, and measuring the byte length.

Before

str       → Content-Length: 4  ✓
bytes     → Content-Length: 4  ✓
BytesIO   → Content-Length: 4  ✓
StringIO  → Content-Length: 1  ✗  (character count, not byte count)

After

str       → Content-Length: 4  ✓
bytes     → Content-Length: 4  ✓
BytesIO   → Content-Length: 4  ✓
StringIO  → Content-Length: 4  ✓

Changes

• src/requests/utils.py: In super_len(), detect io.StringIO and read+encode the remaining text to compute the UTF-8 byte length instead of relying on tell().
• tests/test_utils.py: Added test_super_len_stringio_multibyte covering single emoji, mixed content, partially-read StringIO, and position preservation.

Test plan

• All existing TestSuperLen tests pass (ASCII StringIO, BytesIO, partially-read files, etc.)
• New test verifies correct byte count for multi-byte characters
• New test verifies correct byte count for partially-read StringIO
• New test verifies file position is preserved after super_len() call

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