test: exercise prompt and SSE fixture flow

[sandersonstabo]

Exercises the real test-only child across health, prompt, and SSE connections with bounded I/O, lifeline shutdown, and reap/no-kill evidence.

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Summary by cubic

Adds a seventh fixture scenario to the engine-owner smoke test that runs a real prompt request and SSE log stream against the test-only child, while keeping absolute deadlines and lifeline cleanup.

• The new prompt_text_then_terminal scenario sends an immediate-delivery prompt, then validates the follow-mode SSE events: run_id, sequence numbers 1 and 2, the delta text, and the final succeeded state.
• Shared header and body readers now take a stage label and expected content-type, so timeout and parse failures name the endpoint that produced them.

^{Written for commit 31b9604. Summary will update on new commits.}