fix: add missing f-string prefix in async_to_httpx_files error message

[Quratulain-bilal]

Summary

• Fixed a missing f prefix on the f-string in async_to_httpx_files() (src/openai/_files.py,
  line 100)
• The sync version to_httpx_files() (line 58) correctly uses f"Unexpected file type input {type(files)}...", but the async counterpart was missing the f prefix
• This caused the error message to display the literal string {type(files)} instead of the actual
  type

Bug Details

Before (broken):

raise TypeError("Unexpected file type input {type(files)}, expected mapping or sequence")
# Output: "Unexpected file type input {type(files)}, expected mapping or sequence"

After (fixed):
raise TypeError(f"Unexpected file type input {type(files)}, expected mapping or sequence")
# Output: "Unexpected file type input <class 'int'>, expected mapping or sequence"