Document mutation-testing workflow contract tests

[leynos]

Summary

Adds a "Mutation-testing workflow contract tests" section to
docs/developers-guide.md, documenting the
contract test in tests/workflow_contracts/mutation_testing_test.py that
pins the shape of .github/workflows/mutation-testing.yml.

• Permutation: Rust (single crate) — the caller delegates to
  leynos/shared-actions/.github/workflows/mutation-cargo.yml.
• Contract-test style: shape-only. USES_RE asserts the uses: ref is
  a full 40-hex commit SHA without pinning the value itself, so Dependabot
  bumps it without an accompanying test edit. There is no PINNED_SHA
  constant and no pytestmark skip guard (this is a Rust repo; the
  mutmut-sandbox skip only applies to Python callers).
• Caller inputs documented: only the two the caller actually overrides —
  extra-args: "--all-features" and setup-commands (installs
  clang/lld/mold). paths and exclude-globs are not set, so the
  section notes the shared workflow's defaults apply.
• Local command: make test-workflow-contracts, taken from the
  Makefile target of the same name; confirmed passing locally (6 tests).
• Roadmap/execplan: none applies — the repository has no
  docs/roadmap.md or docs/execplans/ tracking to annotate.
• TOC: docs/developers-guide.md has no internal table of contents to
  cross-link the new section into; docs/contents.md (the docs-set index)
  already lists the developer guide as a single entry, so no change was
  needed there either.

Docs-lint result

make markdownlint (spelling + markdownlint-cli2) passes: 0 errors. Two
spelling nits caught by the repository's en-GB-oxendict policy were fixed
before landing (summarising → summarizing, serialises →
serializes), and the CI step name Install mold linker was moved into a
code span so mold was not flagged as a misspelling of mould.

Test plan

• make test-workflow-contracts — 6 passed (sanity-checks the
  documented local command; not required by this docs-only change).
• make markdownlint — 0 errors.

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