Wire populate runtime selection

[giaphutran12]

Summary

• adds a populate runtime selector for POPULATE_AGENT_RUNTIME / DATASET_AGENT_RUNTIME
• wires both HTTP /populate and populate:self-heal CLI through selected runtime creation
• extracts a testable server factory so the real /populate route can prove selected runtime reaches runSelfHealingPopulate

Verification

• npm --prefix backend test passed: 54/54
• npm --prefix backend run build passed
• make verify-self-healing passed: backend tests, backend build, adapter syntax, and no-key benchmark smoke with zero spend

Notes

• No auto-merge.
• Default runtime remains Mastra.
• POPULATE_AGENT_RUNTIME=collection intentionally requires an injected collection pipeline runner until the real collection pipeline is ported in a later PR.
• Next PR should add the self-healing-wrapped collection benchmark lane.

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