Enable pytest-asyncio debug mode

[llucax]

Fixes #541.

[Marenz]

🤖 The template change and the migration look right, asyncio_debug was
added in pytest-asyncio 1.2.0 and the template pins 1.3.0.

Two things before I approve:

1. This repo has no async tests or fixtures at all, but the PR adds
   pytest-asyncio == 1.4.0 to dev-pytest plus required_plugins,
   asyncio_mode and asyncio_debug to its pyproject.toml. Is that
   deliberate (keeping the config in sync with the template), or can it be
   dropped?

2. The migration doesn't look at the project's pytest-asyncio pin. A repo still
   on < 1.2.0 gets PytestConfigWarning: Unknown config option: asyncio_debug,
   which turns into a failure with the template's filterwarnings = ["error", ...].
   A version check, or at least a hint in the manual step, would avoid that.

Minor: the repo_type == "api" branch is redundant, the generic
"pytest-asyncio" not in content check below already covers it; only
skip-vs-manual-step differs.