Document mutation-testing workflow contract tests

[leynos]

Summary

Adds a ## Mutation-testing workflow contract tests section to
docs/developers-guide.md, documenting the thin caller workflow at
.github/workflows/mutation-testing.yml, its two run modes (daily
change-scoped schedule and whole-crate manual dispatch), the caller's
extra-args/exclude-globs inputs, and what
tests/workflow_contracts/mutation_testing_test.py pins.

• Permutation: A — Rust (single crate).
• Contract-test style: shape-only (USES_RE regex asserting a full
  40-hex commit SHA on mutation-cargo.yml). The SHA value itself is not
  hard-coded, so Dependabot bumps it without any accompanying test edit.
  There is no pytestmark/skip-if guard — this is a Rust repository, so
  the Python sandbox-omits-.github/ concern from the mutmut permutation
  does not apply here.
• Local run command documented: make test-workflow-contracts (the
  Makefile already exposes this target, wrapping
  uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest tests/workflow_contracts -q).
• Only the with: inputs the caller actually sets are documented
  (extra-args: "--all-features" and
  exclude-globs: "src/tests/**,tests/support/**,src/bin/bless_traces.rs");
  there is no paths or extra-crate-dirs input in this caller.

Roadmap/execplan note

docs/roadmap.md and docs/model-training-pipeline-roadmap.md contain no
mutation-testing or workflow-contract-test tracking items, so no
roadmap/execplan annotation applies.

TOC

docs/developers-guide.md has no table of contents or index list near the
top (it is a flat sequence of ## sections), so no cross-link entry was
added; the new section was appended after the existing "Workflow pins and
Dependabot" section, which it complements.

Docs lint

make markdownlint passed (spelling check via make spelling, then
markdownlint-cli2 over all tracked Markdown, 0 errors). Two words in the
initial draft (summarising, serialises) were corrected to the en-GB
Oxford -ize forms (summarizing, serializes) to satisfy the repository's
typos policy. The generated typos.toml was regenerated as a side effect of
running make spelling but reverted before committing, since that churn is
unrelated to this change.

Files changed

• 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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