build(coverage): the ... exclude rule hides whole functions from the coverage report #1681

What happens today

Add a function whose signature contains Callable[..., T], run the test suite, and look for it in the
coverage report. It is not there. Not listed as covered, not listed as missing, simply absent. The same
goes for any block whose opening line contains three dots.

So a reviewer who checks coverage on those functions sees a clean result for code that was never
measured, and the "new code has 100% coverage" item in our PR checklist cannot be verified by our own
tooling.

Why that is a problem

It is silent, and it hides exactly the kind of code we most want measured. In mellea/backends/huggingface.py
the affected functions include the composed local-file, embedded and prompt generation paths, which are
the three places adapter generation actually happens.

How it should behave

Three dots used as a stub body should be excluded, as intended. Three dots inside a type annotation
should not.

Cause

From pyproject.toml, [tool.coverage.report]:

exclude_lines = [
    ...
    "\\.\\.\\.",  # Ellipsis in function bodies
]

exclude_lines entries are regular expressions matched against each source line. When a matched line
opens a block, coverage excludes the whole block. generate_func: Callable[..., _T] matches, so the
function it heads disappears from the report.

Fix

Anchor the pattern to a stub body:

    "^\\s*\\.\\.\\.$",

Then check that the excluded-line count drops and that Protocol and overload stub bodies are still
excluded.

Related

Found while measuring branch coverage during the review of #1654, whose new weightless generation helper
is one of the functions this rule hides.