Add GraphQL ResourceSlotType node with root queries and connections on Agent/Session nodes

[HyeockJinKim]

Add GraphQL node, root queries, and relay connections for resource slot types.

• ResourceSlotTypeGQL(Node) with all columns from resource_slot_types table
• AgentResourceGQL(Node) for agent_resources (agent_id, slot_name, capacity, used)
• ResourceAllocationGQL(Node) for resource_allocations (kernel_id, slot_name, requested, used)
• Root queries: resource_slot_type(slot_name), resource_slot_types(filter, order, pagination)
• Connections on AgentV2GQL: resource_slots -> AgentResourceConnection
• Connections on ComputeSessionGQL/KernelGQL: resource_allocations -> ResourceAllocationConnection
• Shared fetcher pattern for reuse across root queries and node connections

Target Files

New: api/gql/resource_slot/{init,types,fetcher,resolver}.py, api/gql/data_loader/resource_slot/
Modify: api/gql/schema.py, api/gql/agent/types.py, api/gql/session/types.py, api/gql/kernel/types.py, data_loader/data_loaders.py

Success Criteria

• ResourceSlotTypeGQL node exposes all resource_slot_types columns; root query resource_slot_types returns Connection with filter/order/pagination
• Root query resource_slot_type(slot_name) returns single node or null
• AgentV2GQL node has resource_slots field returning AgentResourceConnection (capacity, used per slot)
• ComputeSessionGQL or KernelGQL node has resource_allocations field returning ResourceAllocationConnection (requested, used per slot)
• Fetcher functions are shared between root queries and connection resolvers (no duplication)
• All new types registered in GQL schema and queryable via GraphQL introspection
• pants test passes for affected packages

JIRA Issue: BA-4904