Drop always-id match_field from ListPathSegment runtime metadata

[ellisandrews-toast]

What

Removes the match_field attribute from ListPathSegment in the runtime metadata, and switches the list-vs-object segment discriminator in SourcedFromNestedPathSegment.from_hash from match_field to source_field.

Why

match_field was always "id" — ElasticGraph relationships join on id via foreign keys, so the value carried no information. Serializing a constant into runtime metadata adds noise without value, and keeping it would have meant emitting match_field: "id" into runtime_metadata.yaml once nested sourced paths are actually populated.

This mirrors the equivalent cleanup already applied to the schema-definition PathSegment in #1238. The match is now implicit: list elements are matched on id against the value at source_field. If non-id primary keys are supported in the future, the field can be reintroduced then.

Scope / risk

Low. ListPathSegment is not yet constructed by any production code — only by the round-trip serialization unit test — so there is no schema-artifact churn and nothing downstream consumes the removed field. Specs and type checks pass.