Add MultiEdge count API backed by agg group

[zipdoki]

Summary

Adds a /multi-edges/count endpoint that returns the count of multi-edges for a given (source, target) pair, backed by the existing agg group mechanism.

The counter field (_target for OUT, _source for IN) is automatically prepended to ranges, so callers only need to provide start, target, and direction. Additional ranges (e.g. time bucket) can be appended for period-based counts.

Requires the table schema to declare a COUNT group with _target (or _source) as the leading field.

Endpoint

/graph/v3/databases/{database}/tables/{table}/multi-edges/count\
?start={start}&target={target}&direction={direction}&group={group}&ranges={ranges}

Response

{
  "counts": [
    {
      "start": "userA",
      "direction": "OUT",
      "count": 2,
      "context": {}
    }
  ],
  "count": 1,
  "context": {}
}

1. total count (group defaults to _count)

curl -X GET "http://localhost:8080/graph/v3/databases/{database}/tables/{table}/multi-edges/count\
?start=userA&target=postB&direction=OUT"

2. period-based count

curl -X GET "http://localhost:8080/graph/v3/databases/{database}/tables/{table}/multi-edges/count\
?start=userA&target=postB&direction=OUT&group=day&ranges=time:bt:20240601,20240630"

Changes

• QueryService.multiEdgeCount: builds fullRanges from target + optional ranges and delegates to agg
• MultiEdgeQueryController: exposes GET /multi-edges/count
• MultiEdgeAggCountPayload, DataFrameMultiEdgeAggCountPayload: response payload types
• MultiEdgeCountE2ETest: E2E tests for total count and period-based count
• EdgeMutationBuilder: add MULTI_EDGE_COUNT_GROUP_NAME constant as the default group name for (source, target) count

How to Test

./gradlew :server:test --tests "com.kakao.actionbase.server.api.graph.v3.MultiEdgeCountE2ETest"

AI Assistance

• This PR was written largely with AI assistance.
  • Tool / model: Claude Sonnet 4.6 (claude.ai/code)