Document mutation-testing workflow contract tests

[leynos]

Summary

• Adds a new ## Mutation-testing workflow contract tests section to
  docs/developers-guide.md, documenting the informational mutation-testing
  caller workflow (.github/workflows/mutation-testing.yml) and the contract
  test that pins its shape (tests/workflow_contracts/mutation_testing_test.py).
• Permutation B (Rust workspace): the manual-dispatch run mutates the whole
  workspace, and extra-args carries --test-workspace=true (alongside
  --all-features) so each crate is mutated against the full workspace test
  run, matching the make test baseline.
• Contract-test style: shape-only. USES_RE asserts the pin is a full
  40-hex commit SHA on mutation-cargo.yml, not a specific value, so
  Dependabot bumps it without any accompanying test edit. There is no
  pytestmark skip guard; this is a Rust repository, so .github/ is not
  stripped from any sandbox the test runs in.
• Local run command documented: make test-workflow-contracts (the Makefile
  target exists and wraps
  uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest tests/workflow_contracts -q).
• No table of contents/index list exists inside docs/developers-guide.md
  itself (only ##/### headings), so no in-file cross-link was added.
  docs/contents.md indexes whole documents, not developer's-guide
  subsections, and already lists the developer's guide, so it needed no
  change either.
• No roadmap or execplan tracking applies to this change; checked
  docs/roadmap.md and docs/execplans/* and found no open item referencing
  the mutation-testing workflow or its contract test.

Docs lint

make markdownlint (spelling/typos check plus markdownlint-cli2) passes
cleanly against the full doc set, including the new section.

Test plan

• make markdownlint
• No Rust/Python test suite run (docs-only change, per brief)

Edited file: 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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