Document mutation-testing workflow contract tests

[leynos]

Summary

• Adds a new ## Mutation-testing workflow contract tests section to
  docs/developers-guide.md, explaining
  .github/workflows/mutation-testing.yml, its delegation to
  leynos/shared-actions/.github/workflows/mutation-cargo.yml, and the
  contract test that pins the caller's shape.
• Documents the local run command, make test-workflow-contracts.

Permutation

Permutation A — Rust (single crate). ddlint is a single-crate package (no
workspace), so the section describes a manual dispatch that "mutates the
whole crate".

Contract-test style

Shape-only: tests/workflow_contracts/mutation_testing_test.py uses a
USES_RE regex asserting ...@[0-9a-f]{40}; the pinned SHA value itself is
not hard-coded, so Dependabot bumps it without any accompanying test edit.
The with: block is asserted by equality against a separate EXPECTED_WITH
dict (exclude-globs, extra-args — this repo's caller has no paths
input). The module has no pytestmark/skip guard; ddlint is a Rust crate, so
there is no mutmut sandbox that omits .github/.

Local run command

make test-workflow-contracts, which runs:

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 and
docs/execplans/* were searched for "mutation" and contain no matching task,
so nothing was ticked or annotated.

TOC / cross-link

docs/developers-guide.md has no in-page table of contents (headings run
directly, no index list). docs/contents.md already lists the developer
guide as a whole document ("Maintainer-facing workflow and implementation
guidance for the current codebase"), which already covers this addition, so
no cross-link changes were needed.

Docs lint result

make markdownlint passed after the change (it chains spelling, which
enforces en-GB Oxford-ize spelling via pinned typos). Two Oxford-spelling
fixes were needed in the drafted prose (summarising → summarizing,
serialises → serializes) before the gate passed cleanly. The gate's
spelling step regenerates typos.toml from the shared dictionary as a
side effect; that regeneration was unrelated to this change and was reverted
before committing, leaving typos.toml untouched in this PR.

Only the docs gate was run, per instructions; the Rust/Python test suites
were not run.

Changed file

• 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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