"# E:" lines (etc) in test files are ignored at the beginning of lines

As discovered in the unrelated https://github.com/python/mypy/pull/19494#discussion_r2243899624, "# E:" lines (etc) in test files are ignored at the beginning of lines. Having looked at the code, I've discovered this is because it desires the # to be preceded by a space, which is not present when the # is at the beginning of a line. This is bad because attempted error lines should not be silently ignored. (Although it is unlikely for someone to try to E an empty line.)

My PR #19546 will fix this. Edit: that would still be true, but #21172 is a much more minimal fix for this.

Easy repro:

[case testErrorCodeOnEmptyLine]
# E: This isn't a real message, and thus this test could never pass, although somehow it currently does :)

Beginne mit mypy/test/data.py etwa um Zeile 536 und reproduziere das Verhalten anhand des im Issue gezeigten Testfalls. Vergleiche den vorgeschlagenen minimalen Fix in Issue #21172, da im Body steht, dass der umfassendere Ansatz in PR #19546 nicht mehr bevorzugt wird. Erledigt bedeutet, dass die ungültige „# E:“Assertion am Zeilenanfang nicht mehr stillschweigend ignoriert wird.