Migrate .coveragerc to pyproject.toml section

[rolandwalker]

Description

Covered under an existing changelog entry.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

1. Regression risk: subprocess coverage startup still points to deleted file

• In test/features/environment.py:48, COVERAGE_PROCESS_START is still set to .coveragerc, but this PR deletes that file.
• Impact: behave subprocesses that call coverage.process_startup() can fail to load config (or silently lose intended coverage settings), which can break CI or coverage accounting.
• Fix: set it to pyproject.toml (or restore a compatibility .coveragerc shim).

2. Packaging manifest still references removed file

• MANIFEST.in:2 still includes .coveragerc.
• Impact: sdist/packaging may emit missing-file warnings and indicates migration is incomplete.
• Fix: remove .coveragerc from MANIFEST.in.

Missing test coverage

• There’s no guard test for the behave coverage bootstrap path after this migration. Add a small test asserting the config file referenced by COVERAGE_PROCESS_START exists (or that coverage startup succeeds with the configured path).