Problem matchers which match an invalid file cause annotations to come up as being in the file .github

Describe the bug
If a problem matcher doesn't match a valid file (e.g. the regex captures "test.txt ", with a space at the end, the annotation comes up as being in the file .github rather than the matched file.

To Reproduce
Create a problem matcher which matches at least an invalid filename, or possibly a file not in the repo.

Expected behavior
Either not annotating .github, or annotating the match for a filename that doesn't exist, or perhaps clearest some sort of error somewhere (perhaps in the action log), that the matched filename doesn't exist.

No file or test is named in the report. Reproduce the problem with a matcher whose filename capture has a trailing space or names a missing file, then trace how the toolkit turns that match into an annotation. Done means invalid paths no longer produce a misleading .github annotation, with behavior covered by an appropriate test.