[POC V1] T7.2 Add end-to-end POC smoke test

[fcarucci]

Imported from docs/milestones/proof-of-concept-1/poc_v1_task_breakdown.md.

Phase: P7 End-To-End Proof

Source task: T7.2

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Purpose:

• verify the full vertical slice from UI to adapter

Output:

• one automated smoke test covering app load through graph render

Tests:

• start app with mock adapter
• open dashboard
• verify gateway status is visible
• verify expected agent nodes are visible
• verify expected edges are visible
• verify a metadata-derived edge uses the expected visual distinction

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