Exclude the case line when an irrefutable case body is excluded - #2269

The problem

Excluding the body of a case clause doesn't propagate to the case line, so
the case line is still reported as missing (or as a partial branch under
--branch). This is inconsistent with if/else, where excluding the else:
body makes the else: line disappear from the report.

Reproducer from the issue, with exclude_lines = assert False$:

match 1:
    case 1:
        print("all is well")
    case _:            # still reported as uncovered
        assert False   # excluded

Root cause

An irrefutable case (a wildcard case _:, or a bare capture like
case other:, with no guard) is the match equivalent of an else clause.
Unlike else:, though, the case line is a real statement with its own arc,
so excluding only its body left the case line behind as a missing statement /
partial branch.

The fix

In PythonParser._raw_parse, while walking the AST, if a case is irrefutable
and its entire body is excluded, the case line is added to the excluded set
too. This mirrors the existing handling that carries exclusions from a
decorator/signature onto a function or class body.

The wildcard detection that AstArcAnalyzer._handle__Match already performed is
factored out into a small _case_is_irrefutable() helper that both places now
share.

Refutable cases such as case 2: are deliberately left alone: their case
line stays reported when the body is excluded, the same way an elif line does.

Tests

• tests/test_parser.py: three unit tests covering an irrefutable case _:, a
  bare-capture case other:, and a case whose body is only partly excluded
  (which must not be excluded).
• tests/test_coverage.py: an end-to-end test in branch mode checking that the
  wildcard is excluded while a refutable case 2: with an excluded body is
  still reported as missing.

$ python -m pytest tests/test_parser.py tests/test_coverage.py tests/test_arcs.py
287 passed, 1 skipped

ruff format --check, ruff check, and mypy --strict are clean on the
changed files; pylint reports only a pre-existing unrelated TODO.

Fixes #1563.