Fix mypy plugin loading after airflow_mypy distribution migration

[potiuk]

After #61422 ("Make Mypy plugins installable") two mypy hooks broke
because the plugin references were not migrated consistently:

• devel-common's mypy extra did not depend on apache-airflow-mypy,
  so any hook that pulls apache-airflow-devel-common[mypy]
  (mypy-airflow-core, mypy-task-sdk, mypy-devel-common, …) failed
  with Error importing plugin "airflow_mypy.plugins.decorators": No module named 'airflow_mypy'.
• dev/pyproject.toml referenced the plugins by file path with the
  wrong subpackage name (airflow_mypy/plugin/decorators.py — singular,
  path-style), so mypy-dev failed with Can't find plugin "dev/airflow_mypy/plugin/decorators.py".

Adding apache-airflow-mypy to devel-common's mypy extra and
switching dev/pyproject.toml to the module-style plugin names that
already work in the root pyproject.toml fixes both.

Verified locally:

• prek run mypy-devel-common --all-files — Passed
• prek run mypy-dev --all-files — Passed
• prek run mypy-airflow-core (one file) — Passed

---

Was generative AI tooling used to co-author this PR?

• Yes — Claude Opus 4.7 (1M context)

Generated-by: Claude Opus 4.7 (1M context) following the guidelines

[boring-cyborg]

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[github-actions]

Backport failed to create: v3-2-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-2-test

You can attempt to backport this manually by running:

cherry_picker c43c778 v3-2-test

This should apply the commit to the v3-2-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.