Fix over-quoting of backslashes in table cells

[golikovichev]

Fixes #769

Problem

Backslashes in datatable and Examples table cells were doubled. A cell written as a single backslash reached the step as two:

Then expect backslash in datatable
  | \ |

@then("expect backslash in datatable")
def _(datatable):
    assert datatable[0][0] == "\\"   # failed: value was "\\\\"

Cause

gherkin_parser.Cell.from_dict ran every cell value through a helper:

def _to_raw_string(normal_string: str) -> str:
    return normal_string.replace("\\", "\\\\")

The gherkin library already resolves cell escaping per the Gherkin spec before pytest-bdd sees the value (\\ -> \, \| -> |, \n -> newline, a lone \ stays \). The extra pass doubled every backslash a second time. This looks like the leftover compatibility workaround mentioned in the issue thread.

Fix

Deliver the cell value exactly as gherkin resolves it and drop the now-unused helper (it had a single call site).

Backwards compatibility

This is a behaviour change, not a pure internal fix. Cells now carry the Gherkin-resolved value with no extra doubling. Anyone who relied on the old doubled output (for example asserting \\ for a single authored backslash, or writing C:\\\\Users to survive the old double pass) will need to drop that workaround. Values now match what the feature author wrote and what other Gherkin implementations produce.

Tests

• New tests/datatable/test_datatable.py::test_datatable_preserves_backslashes covering a lone backslash, an escaped \\, an escaped pipe \|, and a Windows path C:\Users\John. Expected values were checked against the raw gherkin parser output.
• Updated the two test_outline_with_escaped_pipes expectations that encoded the old doubled output (bork \\ now decodes to a single trailing backslash; bork \\\| now decodes to bork \|).
• CHANGES.rst: entry under Unreleased / Fixed.

Full suite passes locally except for pre-existing Windows-only path-separator failures in test_generate / test_migrate / test_feature_base_dir that also fail on an unmodified checkout and are unrelated to this change.