Document mutation-testing workflow contract tests

[leynos]

Summary

• Adds a new ## Mutation-testing workflow contract tests section to
  docs/developers-guide.md, placed after the
  existing "Workflow pins and Dependabot" section and before
  "Property-based testing".

• Documents the thin caller
  .github/workflows/mutation-testing.yml,
  which delegates to leynos/shared-actions/.github/workflows/mutation-mutmut.yml
  (Permutation C: Python/mutmut), the paths and module-prefix-strip
  inputs it actually sets, and the [tool.mutmut] configuration in
  pyproject.toml.

• Documents the contract test
  tests/workflow_contracts/test_mutation_testing.py:
  it is shape-only (a USES_RE regex asserting a full 40-hex commit SHA,
  not a hard-coded value, so Dependabot bumps freely), it self-skips via
  pytestmark = pytest.mark.skipif(...) when the workflow file is absent
  (mutmut's sandbox omits .github/), and it validates the uses: pin, the
  with: block, job/workflow permissions, concurrency, and triggers.

• Documents the local run command, verified to pass:

  uv run pytest tests/workflow_contracts/test_mutation_testing.py -v

  (6 passed). There is no make test-workflow-contracts target in this
  repository's Makefile, so the direct pytest invocation is documented
  instead.

Roadmap / execplan note

Checked docs/roadmap.md and docs/execplans/; neither contains any
mutation-testing or contract-test tracking item, so no roadmap/execplan
tracking applies to this change.

Table of contents note

docs/developers-guide.md has no internal table of contents or section
index near the top (it opens with prose and then runs straight into
headings), so no cross-link was added there. docs/contents.md is a
whole-repository documentation index and already links to
developers-guide.md as a single entry; it does not enumerate the guide's
internal sections, so it was left unchanged.

Docs-lint result

Ran make markdownlint (the repository's Markdown lint + spelling gate,
which chains spelling then markdownlint-cli2). It initially caught two
en-GB-oxendict -ise/-ize violations introduced by the new section
(summarising -> summarizing, serialises -> serializes); both were
fixed and the gate now passes cleanly (0 errors). The Rust/Python test
suites were intentionally not run, per the docs-only scope of this change.

Files changed

• docs/developers-guide.md

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✨ Finishing Touches

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

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