Fix escaped trailing pipes being removed from table cells

[walitemuri]

With the tables extra, an escaped pipe at the end of a row is stripped as if it were the optional closing table border. For example:

Name | Operator \|
--- | ---
or | \|

The heading renders as Operator \ and the data cell as \, instead of Operator | and |. _split_row now preserves an escaped final pipe so the existing cell unescaping can handle it.

The regression fixture covers headers and body rows, optional leading/trailing borders, repeated escaped pipes, alignment, emphasis, and empty cells. It fails before the implementation change and passes afterward.

Validation on macOS with Python 3.13 and all optional dependencies installed:

• cd test && ../.venv/bin/python test.py -- -knownfailure: 270 tests pass. The runner retains its standard known-failure and Pygments-version exclusions.
• .venv/bin/python test/test_redos.py: all 7 checks pass.
• pycodestyle --diff for the changed Python lines and git diff --check: pass.

Prepared with OpenAI Codex assistance.

[nicholasserra]

Thanks! If you can go in and fix the CHANGES format i'll hit merge

[walitemuri]

Fixed

[nicholasserra]

Thanks!