Add parent_relationship API and RelationshipChainResolver with validation

[ellisandrews-toast]

Summary

ElasticGraph's sourced_from feature allows fields on an indexed type to be populated from events published by a different source type. Today, this only works for top-level indexed types.

This PR is an incremental step toward supporting sourced_from on embedded (non-indexed) types — types that live nested inside a root document.

To enable this, we introduce a parent_relationship API that lets users declare how an embedded type connects back up to the root indexed type.

This PR adds the API and validates the chain of parent_relationship declarations — producing clear error messages for misconfigured schemas. Valid configurations are artifact no-ops for now; later PRs will use the validated chain to produce runtime metadata that tells the indexer how to navigate into nested documents and update the correct fields.

Example API Usage

ElasticGraph.define_schema do |schema|
  schema.object_type "Team" do |t|
    t.field "id", "ID!"
    t.field "name", "String"
    t.field "players", "[Player!]!" do |f|
      f.mapping type: "nested"
    end
    t.relates_to_many "statLines", "StatLine", via: "teamId", dir: :in, indexing_only: true
    t.index "teams" do |i|
      i.has_had_multiple_sources!
    end
  end

  schema.object_type "Player" do |t|
    t.field "id", "ID!"
    t.field "name", "String"
    t.field "goalsScored", "Int" do |f|
      f.sourced_from "statLine", "goals"
    end
    t.relates_to_one "statLine", "StatLine", via: "playerId", dir: :in, indexing_only: true do |r|
      r.parent_relationship "Team", "statLines" # <--- NEW!
    end
  end

  schema.object_type "StatLine" do |t|
    t.field "id", "ID!"
    t.field "teamId", "ID"
    t.field "playerId", "ID"
    t.field "goals", "Int"
    t.index "stat_lines"
  end
end

Validations

• Relationship must be indexing_only: true
• No circular parent chains
• Parent type must exist
• Parent relationship must exist on parent type
• Source types must agree across the chain
• Chain must terminate at an indexed type
• Embedding field must exist on parent type (auto-discovered or explicit via parent_field_name:)
• No ambiguous embedding (multiple fields of the same type without parent_field_name:)

[myronmarston]

I'm not done reviewing but I wanted to submit my feedback so far.

[myronmarston]

LGTM--just one minor suggestion!