Exclude ... bodies after multi-line return-type annotations - #2196

The default exclusion regex for ... function bodies expects the def
signature to end with ). When a formatter (e.g. Ruff) splits a long
return-type annotation across multiple lines, the last line of the
signature ends with ] instead — for example:

class C:
    def f() -> dict[
        str, str
    ]: ...

With the current regex this function's body is reported as a missing
line, so coverage drops below 100% even though the body is just ....

This change widens the optional def-header group so the closing
bracket can be any (non-empty) run of ] and ). That lets the
regex still match:

• def f(): ...
• def f() -> int: ...
• multi-line def signatures that close with ): ... (existing case)
• multi-line def signatures that close with ]: ... (the bug)
• multi-line def signatures that close with ]) -> int: ...
• Tuple[int, ...] parameter types whose inner ... should not
  itself trigger the rule

It does not start matching unintended things: an annotated assignment
like x: list[int] = ... still doesn't match (no : after the ]),
and a regular function with a real body doesn't match either.

End-to-end reproducer from #2185:

$ cat example.py
class C:
    def f() -> dict[
        str, str
    ]: ...
$ coverage run --branch -m example && coverage report --fail-under=100

Before the change: example.py 3 0 2 1 80% (fail-under trips).
After: example.py 2 0 0 0 100%.

Testing

• Added ExcludeTest::test_default_ellipsis_multiline_return_type covering
  single-line, split-return, split-param-and-return, and inner-Tuple[..., ...]
  shapes.
• Existing ExcludeTest::test_default and the rest of ExcludeTest still
  pass (7/7).
• tests/test_coverage.py tests/test_config.py tests/test_parser.py —
  233 passed, 7 skipped.
• ruff format --check and mypy --strict on the changed files — clean.

Fixes #2185.