Fixed the issue where field values containing spaces were being rejected with an unhelpful error message (#1665)

[SandeepaInduwaraSamaranayake]

The Issue @d0niek reported #1665:

http --print=B POST api.url.com field_name="Foo Bar"

Would fail with the error:

error:
    argument REQUEST_ITEM: 'Bar' is not a valid value

This happens when shell argument parsing splits the improperly quoted arguments, causing 'Bar' to be passed as a separate argument without a separator (=, :, @, etc.).

The Fix:

Improved the error handling in to provide a more helpful error message:

# Before
else:
    raise argparse.ArgumentTypeError(f'{s!r} is not a valid value')

# After  
else:
    # Check if this looks like it should be a continuation of a previous value
    # (i.e., it's a string without any separator, likely due to unquoted shell args)
    if all(c not in s for c in self.special_characters):
        raise argparse.ArgumentTypeError(
            f'{s!r} is not a valid value. Did you mean to quote it as part of a previous item? '
            f'For example: field="value {s}"'
        )
    raise argparse.ArgumentTypeError(f'{s!r} is not a valid value')

Added comprehensive tests in to cover:

1. test_invalid_item_without_separator_provides_helpful_error - Ensures the helpful error message appears

2. test_values_with_spaces_when_properly_quoted - Tests three scenarios with properly quoted values containing spaces:

          - field=value with spaces
          - field_name="Foo Bar"
          - data=Hello World Test

Verification

All 42 tests in test_cli.py pass
Values with spaces now work correctly when properly quoted
Users get a helpful error message when forgetting quotes instead of a confusing "not a valid value" message
The original use case works as expected:

http --print=B POST httpbin.org/post field_name="Foo Bar"
# Returns: {"field_name": "Foo Bar"}