[libgraphql-core-v1] Task 16: Schema struct, SchemaBuilder::build(), OperationKind

[jeffmo]

Summary

The capstone task that makes the schema pipeline usable end-to-end: parse → load → build → query.

Schema struct

• Full typed query API: get_type(), object_type(), interface_type(), enum_type(), scalar_type(), union_type(), input_object_type()
• Typed iterators: object_types(), interface_types(), enum_types()
• types_implementing(interface_name) — finds all types implementing a given interface
• Root operation types: query_type(), mutation_type(), subscription_type() (with name accessors)
• Collection accessors: types(), directive_defs(), source_maps()
• Serde-serializable for macro crate embedding

SchemaBuilder::build()

5-phase orchestration:

1. Resolve root query type — explicit schema { query: ... } or implicit "Query" default per spec
2. Validate root types — must exist and be Object types
3. Empty type checks — EmptyObjectOrInterfaceType, EmptyUnionType, EnumWithNoValues (build-level SchemaBuildErrorKind variants)
4. Run validators — ObjectOrInterfaceTypeValidator, UnionTypeValidator, InputObjectTypeValidator, validate_directive_definitions
5. Produce Schema or SchemaErrors

OperationKind enum (pulled forward from Task 18)

• Query, Mutation, Subscription with Display and From<libgraphql_parser::ast::OperationKind>
• Used in error variants instead of String for type safety

Test plan

• 14 new end-to-end tests: minimal build, all type kinds, no query type, non-object root, empty types, invalid interface impl, typed lookups, convenience API, implicit/explicit query resolution, mutation/subscription roots, directive lookups, missing mutation root, empty enum
• 208 total tests pass
• cargo clippy --tests -- -D warnings clean

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