Document mutation-testing workflow contract tests

[leynos]

Summary

• Adds a "Mutation-testing workflow contract tests" section to
  docs/developers-guide.md, documenting the
  .github/workflows/mutation-testing.yml caller (delegating to
  leynos/shared-actions/.github/workflows/mutation-cargo.yml) and the
  contract test that pins its shape,
  tests/workflow_contracts/mutation_testing_test.py.
• This is a docs-only change; no workflow or test behaviour is altered.

Permutation

Permutation A (Rust, single crate). The caller's actual with: block sets
extra-args: "--all-features" and setup-commands (installing
clang/lld/mold); it has no paths or exclude-globs inputs, so those
were omitted rather than invented. The contract test is shape-only
(USES_RE regex asserting a 40-hex-character commit SHA) — Dependabot bumps
the pin freely with no accompanying test edit, consistent with the existing
"Workflow pins and Dependabot" guidance later in the same document.

Local run command

make test-workflow-contracts, confirmed against the real Makefile target
(uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest tests/workflow_contracts -q).

Roadmap/execplan note

No roadmap or execplan tracking applies: docs/roadmap.md has no
mutation-testing entries, and the repository has no docs/execplans/
directory.

TOC/cross-link note

docs/developers-guide.md has no in-document table of contents or section
index to update. docs/contents.md (the documentation-set index) already
describes the developer guide generically ("explains the local workflow and
implementation tooling for contributors"), which remains accurate without
changes.

Docs lint

make markdownlint (which chains spelling checks and markdownlint-cli2)
passes: 0 error(s). Two en-GB Oxford spellings introduced in the first
draft ("summarising", "serialises") were corrected to "-ize" forms
("summarizing", "serializes") to satisfy the repository's typos
configuration.

Test plan

• make markdownlint passes with the new section included.
• No code or workflow test suites were run, per the docs-only scope of
  this change.

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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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