Add synchronous /execute endpoint for ML-Commons connector

[mingshl]

Originally created by @mingshl in mingshl/opensearch-agent-server#18

Description

Add a synchronous POST /execute endpoint that returns a JSON response (not SSE). This is needed for the ML-Commons remote agent connector, which expects a request-response pattern rather than streaming.

Background

The ML-Commons plugin in OpenSearch can call remote agents via a connector. The connector expects a synchronous HTTP endpoint that returns a complete JSON response.

API Design

POST /execute
Content-Type: application/json

{
  "input": "user query",
  "context": [...]
}

Response:
{
  "output": "agent response text",
  "tools_used": [...],
  "metadata": {...}
}

Implementation

• New route in src/server/ (e.g., execute_routes.py)
• Reuse existing agent factory and orchestrator
• Collect all events internally, return final text as JSON
• Respect auth middleware and rate limiting
• Add to OpenAPI docs

References

• Current SSE endpoint: src/server/run_routes.py