chore(ci): align test jobs with mise tasks

[einarwar]

Make the test jobs use the same mise-action + setup-uv + mise run <task> pattern already established by the mypy and typescript-compile jobs in linting-and-checks.yaml, so local mise run <task> and CI behave identically.

Changes

• Add .mise/tasks/test.toml with a test:api:unit task (cd api && uv run pytest --unit, depends on install-dependencies:api).
• Rewrite api-unit-tests to use mise-action + setup-uv + mise run test:api:unit.
• Rewrite docs-tests to use mise-action + setup-uv + mise run docs:build (existing task). Renamed the misleading "Checkout GitHub Action" step name and dropped the commented-out fetch-depth/if lines.
• Drop unused CR_SECRET workflow_call input — it was declared but never referenced in the file, and no caller passes it.
• Move API_IMAGE env into the api-integration-tests job (its only consumer).
• Rename dead WEB_IMAGE to NGINX_IMAGE in the (currently disabled) web-tests job, matching the image actually published by publish-image.yaml (ghcr.io/equinor/template-fastapi-react/nginx). Scoped to that job.

Notes / out of scope

• Considered also dropping the mypy / typescript-compile jobs as redundant with pre-commit, but .pre-commit-config.yaml does not include mypy or tsc — so those jobs are not redundant. Dismissed.
• Considered converting the raw docker pull + docker build steps in api-integration-tests / web-tests to docker/build-push-action. Deferred to a follow-up PR that will also cover publish-image.yaml.
• web-tests remains if: \${{ false }} (separate cleanup item).