Fix #to_json for IO objects, fixes #26132

[rohitpaulk]

Summary

Fixes #26132.

Before

• For unreadable IO objects, to_json would raise an IOError
• For readable IO objects, to_json would cause a side-effect by reading the contents of the stream.

After

IO#to_json returns the to_s representation for both readable and unreadable IO objects.

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