[Enhancement - Junior Level] Recognize PostgreSQL \\N as a missing-value sentinel

[JohnnyWilson16]

The Problem: SQL and warehouse exports often use \\N to represent NULL. FreshData already handles many missing markers, but this common export format should be cleaned automatically.

The Solution: Add "\\N" support to the default sentinel registry so values like "\\N" and " \\N " are treated as missing only when the entire cell matches.

Acceptance Criteria:

• Update src/freshdata/_sentinels.py with the new sentinel.
• Add tests in tests/test_strings.py.
• Confirm "\\N" and " \\N " become missing.
• Confirm partial values like "A\\N42" are preserved.
• python -m pytest tests/test_strings.py passes.

Skills Required: Python, PyTest, Pandas, string normalization

How to Get Started:

1. Open src/freshdata/_sentinels.py and read the existing sentinel list.
2. Open src/freshdata/steps/strings.py to see how matching works.
3. Add the smallest possible tests in tests/test_strings.py before changing behavior.

[WilliamK112]

Implemented and opened PR for this issue.

What I changed:

• added "\\N" to src/freshdata/_sentinels.py defaults.
• added targeted regressions in tests/test_strings.py:
  • \N + whitespace-padded \N are treated as missing
  • A\N42, \N42, 42\N are preserved (no partial matching)

Validation:

• python -m pytest tests/test_strings.py --no-cov (pass)

PR: #25