Add get_duo_session mcp tool

What does this MR do and why?

Adds the EE get_duo_session MCP tool, with the legacy get_duo_workflow_status alias, so MCP clients can retrieve a Duo Agent platform session by numeric workflow ID.

References

closes #607634 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Create an OAuth token that can access the MCP server, then set the endpoint, token, and a numeric workflow ID visible to that user:
2. Check that get_duo_session is registered

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' |
  jq -er '.result.tools[] | select(.name == "get_duo_session") | .name'

3. Retrieve the Duo session

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data "$(jq -cn --argjson workflow_id "$WORKFLOW_ID" \
    '{jsonrpc:"2.0",id:2,method:"tools/call",
      params:{name:"get_duo_session",arguments:{workflow_id:$workflow_id}}}')" |
  jq

4. Validate the response shape

curl --silent --request POST "$MCP_URL" \
  --header "Authorization: Bearer $MCP_OAUTH_TOKEN" \
  --header 'Content-Type: application/json' \
  --data "$(jq -cn --argjson workflow_id "$WORKFLOW_ID" \
    '{jsonrpc:"2.0",id:3,method:"tools/call",
      params:{name:"get_duo_session",arguments:{workflow_id:$workflow_id}}}')" |
  jq -e --argjson workflow_id "$WORKFLOW_ID" '
    .result.isError == false and
    (.result.structuredContent.workflow_id == $workflow_id) and
    (.result.structuredContent.status | type == "string") and
    (.result.structuredContent.web_url | type == "string") and
    ((.result.structuredContent.poll_after_seconds? == null) or
     (.result.structuredContent.poll_after_seconds | type == "number")) and
    ((.result.structuredContent.awaiting_approval? == null) or
     (.result.structuredContent.awaiting_approval | type == "boolean")) and
    ((.result.structuredContent.turn_complete? == null) or
     (.result.structuredContent.turn_complete | type == "boolean"))
  '

5. Run the focused specs

bundle exec rspec \
  ee/spec/services/mcp/tools/duo_workflows/get_duo_session_tool_spec.rb \
  ee/spec/services/mcp/tools/duo_workflows/get_duo_session_service_spec.rb \
  ee/spec/services/ai/duo_workflows/mcp_config_service_spec.rb \
  ee/spec/requests/api/mcp/handlers/list_tools_spec.rb

MR acceptance checklist

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Related to #607634 (closed)