Document mutation-testing workflow contract tests

[leynos]

Summary

Adds a new "Mutation-testing workflow contract tests" section (## 37.) to
docs/developers-guide.md, documenting the caller workflow
.github/workflows/mutation-testing.yml
and its contract test, tests/workflow_contracts/mutation_testing_test.py.

The section covers:

• the shared reusable workflow the caller delegates to
  (mutation-cargo.yml), the two run modes (daily change-scoped schedule,
  whole-workspace manual dispatch), and the informational (non-gating)
  nature of the run;
• the real with: inputs the caller sets — paths, exclude-globs,
  extra-args (including the --test-workspace=true --test-tool=nextest
  flags and the schema_helpers_ui trybuild exclusion), shard-count: 8,
  and setup-commands — with no invented inputs;
• the contract test's shape-only SHA style (a USES_RE regex asserting
  a full 40-hex commit SHA, not a hard-coded pin), its
  pytest.mark.skipif self-skip guard for mutation sandboxes that omit
  .github/, and the local run command, make test-workflow-contracts;
• a note distinguishing the mutation contract test from the separate
  codescene_coverage_test.py, which pins a different action at its own
  SHA, so the two are not conflated.

This repository's docs/developers-guide.md has no table of contents or
index list near the top (sections are plain numbered ## headings), so no
cross-link entry was added; the new section is simply the next numbered
heading in sequence.

No roadmap or execplan tracking applies: docs/roadmap.md has no
mutation-testing items, and docs/execplans/mutation-testing.md is already
marked IMPLEMENTED for the original (now superseded) workflow shape — it
is not a checklist for this documentation addition.

Docs lint

make markdownlint (spelling/typos + markdownlint-cli2 on changed
Markdown files) passes with no errors on the final version of this change.
Only the Rust/Python test suites were skipped, per this being a docs-only
change.

Test plan

• make markdownlint passes
• Rust/Python test suites intentionally not run (docs-only change)

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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