Document mutation-testing workflow contract tests

[leynos]

Summary

• Add a ## Mutation-testing workflow contract tests section to
  docs/developers-guide.md, documenting tests/test_workflow_contract.py,
  which pins the shape of .github/workflows/mutation-testing.yml (the caller
  of leynos/shared-actions/.github/workflows/mutation-mutmut.yml).
• The section covers: the caller's with: inputs actually present (paths: hooks/, module-prefix-strip: ""); the shape-only SHA assertion (the
  USES_RE regex, not a hard-coded PINNED_SHA), so Dependabot bumps need no
  accompanying test edit; the pytestmark = pytest.mark.skipif(not WORKFLOW_PATH.exists(), ...) guard that self-skips the test inside
  mutmut's mutants/ sandbox (which omits .github/); and the local run
  command.
• Placed as a new top-level section immediately after the existing "Workflow
  pins and Dependabot" section, since both discuss the same shape-only pin
  discipline.

Permutation

Permutation C (Python/mutmut), reconciled to this repository's actual values:
mutation scope is hooks/ only, module-prefix-strip is empty (flat
layout), and no exclude-globs/extra-args are set (both default upstream).

Local run command

No make test-workflow-contracts target exists. The documented command
matches the Makefile's PYTEST variable:

uv run --group dev python -m pytest tests/test_workflow_contract.py -v

Roadmap / execplan tracking

No roadmap or execplan tracking applies. ROADMAP.md has no mutation-testing
item, and docs/execplans/adopt-vendcurl.md is unrelated.

Cross-linking

docs/developers-guide.md has no table of contents or index list, so no
cross-link was added.

Docs lint

bash markdownlint docs/developers-guide.md — 0 errors (after wrapping one
86-column line to fit the 80-column limit). make spelling's underlying
checks (typos_rollout_cli.py generate/check plus typos --config typos.toml) ran clean against the new section.

File changed

• docs/developers-guide.md

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/mutation-contract-tests

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