Preserve escaped test section markers in update-data

[popsiclelmlm]

Summary

• Preserve escaped .test data lines when --update-data rewrites inline error comments.
• Add a regression test for an escaped [mypy] config section inside a [file mypy.ini] block.

Reproduction

A .test case can include literal section-like lines by escaping the opening bracket, for example:

[file mypy.ini]
\[mypy]
warn_unused_ignores = True

Before this change, running pytest --update-data on a case that touched that file section rewrote the line as [mypy], dropping the escape.

Root cause

parse_test_data unescapes special test-data lines before returning TestItem.data; --update-data then wrote those parsed source lines back directly. That is correct for executing the test case, but not for serializing it back to .test syntax.

Changes

• Add a small serialization helper for update-data rewrites.
• Re-escape source lines that start with [ so they stay literal data instead of becoming test section headers.
• Re-escape source lines that start with --, matching the existing parser escape convention for literal command-like lines.
• Cover the bracket case in mypy/test/meta/test_update_data.py.

Tests

• /tmp/mypy-update-data-venv/bin/python -m pytest -q mypy/test/meta/test_update_data.py
• git diff --check
• /tmp/mypy-update-data-venv/bin/python -m mypy --config-file mypy_self_check.ini mypy/test/update_data.py mypy/test/meta/test_update_data.py

Screenshots/Logs

Not applicable; test infrastructure change only.

Fixes #17465