Add OnCall types and Service.GetOnCalls for incident routing

[saditya370]

Summary

Surfaces the currently on-call person(s) for a service via the Go SDK. Required so the OpsLevel MCP server can answer incident-response questions like "who's on-call for the payments service right now?". Pairs with the backend PR that exposed Service.onCalls publicly on the GraphQL schema (linked below).

Part 2 of OpsLevel GitLab issue 14711 (gap 1 — users tool tags/teams — was #614).

Changes

on_call.go (new)

• OnCall — {ExternalEmail, Id, Name, User *UserId}. Matches the deployed GraphQL OnCall type.
• OnCallConnection — standard connection with Edges, Nodes, PageInfo, TotalCount.
• OnCallEdge — {Cursor, Node *OnCall}.

service.go

• New field on Service: OnCalls *OnCallConnection with graphql:"-" — intentionally excluded from auto-fetch in ListServices to avoid N+1 (the backend has an open follow-up on the user resolver inside OnCallType, flagged during the schema PR review).
• New method Service.GetOnCalls(client, variables) (*OnCallConnection, error). Standard per-service fetcher with pagination follow-through, mirroring Service.GetTags style.

service_test.go

• TestGetServiceOnCalls — mock query + response with two on-call nodes (one mapping to an OpsLevel user, one with only externalEmail), asserts field population and nil-handling for User.
• TestGetServiceOnCallsInvalidServiceId — asserts the empty-id guard error.

Design decisions