Tests mutate tracked fixture files (scenarios/test_pipelines/*.yaml) on every run

[jeremymanning]

Found while verifying an unrelated change.

Evidence

Running the suite rewrites tracked files in place:

 M tests/scenarios/test_pipelines/concurrent_network.yaml
 M tests/scenarios/test_pipelines/network_recovery.yaml

The diff is an ephemeral port number being substituted into the fixture:

-            url: "http://localhost:58202/test1"
+            url: "http://localhost:63696/test1"

Impact

• git status is dirty after any test run, so real changes are easy to lose in the noise — this actually caused churn to be swept into unrelated commits during this session.
• The committed port number is meaningless: it records whatever ephemeral port one machine happened to get.
• Any CI job that checks for a clean tree will fail spuriously.

Proposed fix

The test should render the fixture to a temp path (tmp_path) with the port substituted, and leave the tracked file untouched. The tracked fixture should carry a placeholder (e.g. {{ port }} or a sentinel) rather than a real port number.

Acceptance criteria

• git status --porcelain is empty after a full test run
• The tracked fixtures contain no machine-specific port numbers