Raise MissingFileSystem instead of AttributeError when diffing reports without source files

[Eljees]

Fixes #171.

Problem

Diffing two reports that were constructed without a filesystem crashes with

AttributeError: 'NoneType' object has no attribute 'has_file'

Cobertura.__init__ leaves self.filesystem as None when no source is given, and CoberturaDiff.file_source() reaches straight past the public accessor into self.coberturaN.filesystem.has_file(filename).

Why the existing error path was unreachable

Cobertura already has a dedicated exception and a helpful message for exactly this situation — Cobertura.MissingFileSystem, raised by _raise_MissingFileSystem() and used by file_source():

...you need to provide the source code directory or an archive...

CoberturaDiff.file_source() simply bypassed it by touching .filesystem directly, so the user got a bare AttributeError from deep inside the library instead of the message the author had already written.

Fix

Add Cobertura._filesystem_has_file(), which raises MissingFileSystem when there is no filesystem and otherwise delegates to it, and call it from the two places in CoberturaDiff.file_source() that reached into .filesystem directly. No new semantics: this only routes an existing crash into the existing error path.

Tests

test_diff_file_source_without_filesystem in tests/test_cobertura_diff.py, parametrised over a file present in both reports and one present in only one of them. It fails on master with the AttributeError above and passes with the change.

The test builds Cobertura directly rather than through tests/utils.py::make_cobertura, because that helper always passes a filesystem — going through it, the test would be green either way.

Also in this PR

• CHANGES.md entry under ## Unreleased.
• README.md: the TextReporterDelta example constructs both Cobertura objects without a filesystem and then calls delta.generate(), which is precisely the crash this PR fixes — so the example as written could not have worked. Updated it to pass filesystem_factory('source1'/'source2'), matching the pycobertura diff invocation shown just above it.

black leaves every file in this PR unchanged.

AI-assisted: I used Claude to help trace the call path and to draft the patch and the test. I reviewed every line, ran the new test against unpatched master first to confirm it fails without the fix, and ran the suite locally; the analysis and the runs are mine.

[Eljees]

Ping — this has been open since 6 August with no review yet.

Diffing two reports built without a filesystem crashes with AttributeError: 'NoneType' object has no attribute 'has_file', because CoberturaDiff.file_source() reaches past the public accessor straight into self.coberturaN.filesystem. Cobertura already has a dedicated MissingFileSystem exception with a helpful message for exactly this case — it was simply being bypassed. The change adds Cobertura._filesystem_has_file() and calls it from the two places that reached in directly, so no new semantics, just an existing crash routed into the existing error path.

build is green on 3.9 through 3.13 for 8e84e9c. The new test fails on master with the AttributeError and passes with the change.

Happy to close it if you would rather handle #171 another way.