Sample MAB router for provider selection

Called by the cloud runner at session start to ask the platform's multi-armed bandit router which provider to use. The platform reads Beta-distribution posteriors and runs Thompson sampling over the caller-supplied candidates list. Returns `null` provider with a `source` reason on all error / disabled / explicit-override paths (HTTP 200 always) so the runner can always fall back to its statically resolved provider. Kill-switch: `ROUTING_SELECTOR_ENABLED` env must be exactly `"true"`; default is DARK. Explicit dispatch overrides always return `{ selectedProvider: null, source: 'explicit' }`.

curl -X POST "https://example.com/api/sessions/string/routing-select" \  -H "Content-Type: application/json" \  -d '{    "workType": "string",    "candidates": [      "string"    ]  }'

{
  "selectedProvider": "string",
  "source": "disabled",
  "expectedReward": 0,
  "confidence": 0
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}