Add tests that keep the platform contract guide aligned with the API

[MuttleyD]

Context

MIDAS now has a consolidated platform contract guide at:

docs/core/platform-contract.md

This guide describes the core MIDAS API, YAML resource model, runtime evaluation contract, evidence contract, and eventing contract.

Because this document describes the platform contract, it is important that it does not drift away from the actual OpenAPI spec, HTTP routes, and example bundles.

Scope

Add a small documentation validation test that checks key references in docs/core/platform-contract.md against source-of-truth files.

The test should verify that the guide references real files and documented paths, including:

api/openapi/v1.yaml
docs/api/http-api.md
docs/core/runtime-evaluation.md
docs/control-plane.md
docs/core/authority-model.md
docs/core/envelope-integrity.md
docs/core/data-model.md
docs/operations/events.md
docs/guides/quickstart-first-evaluation.md
examples/control-plane/

If practical within the existing test style, also verify that key endpoint strings used in the guide exist in the OpenAPI spec or HTTP route definitions, such as:

/v1/evaluate
/v1/controlplane/plan
/v1/controlplane/apply
/v1/evidence/envelopes
/v1/decisions/request
/healthz
/readyz
/metrics

Recommended area to inspect first:

docs/core/platform-contract.md
api/openapi/v1.yaml
internal/httpapi/
internal/controlplane/
existing doc/link tests, if any

Acceptance Criteria

• A lightweight test validates that important links or file references in docs/core/platform-contract.md resolve.
• The test validates a small set of core endpoint references against OpenAPI or route source if there is an existing pattern for doing so.
• The test is narrow and maintainable.
• The test does not require a running MIDAS server.
• The test does not require network access.
• The test uses existing repository test conventions.
• go test ./... or the relevant package test passes, depending on the repository’s existing test structure.
• git diff --check passes.

Out of Scope

• Building a full documentation linter.
• Generating documentation from OpenAPI.
• Changing API routes.
• Changing runtime behaviour.
• Adding external dependencies.
• Adding CI infrastructure.