Replace abstract summarize-nextflow scenarios with concrete pinned fixtures

## Context

Review of #528 found that `content/molds/summarize-nextflow/scenarios.md` contains a mixture of pinned fixture paths and abstract descriptions such as “a synthetic tree,” “each pipeline,” or “any pipeline.” Scenario contracts should name materializable, reproducible fixtures.

## Work

- Review every scenario in `content/molds/summarize-nextflow/scenarios.md`.
- Replace abstract fixture descriptions with a concrete path pinned through `workflow-fixtures/fixtures.yaml` or a committed package-test fixture.
- Where no suitable fixture exists, add the smallest dedicated fixture first and name it explicitly in the scenario.
- Preserve or sharpen each expected result so it identifies observable fields and values.
- Verify the updated scenarios against the summarize-nextflow package tests and casts.

## Scope note

The three test-selection scenarios added by #528 are being made concrete in that PR. This issue covers the pre-existing scenario file comprehensively.

Raised from review comment: https://github.com/galaxyproject/foundry/pull/528#discussion_r4019502681

Review every scenario in content/molds/summarize-nextflow/scenarios.md alongside the mappings in workflow-fixtures/fixtures.yaml; start by running the summarize-nextflow package tests and casts. Replace abstract descriptions with pinned or committed fixtures, add a smallest dedicated fixture where necessary, and preserve expected observable fields and values.

Written by the indexing model from the issue text.