Validate schema root operation types

[phdoerfler]

Fixes #174.

Problem

A schema with no query root type — either because it has no type Query at all and no explicit schema { } block, or because an explicit schema { } block references a root type that doesn't exist — currently either:

• crashes with an unhelpful NoSuchElementException the first time .queryType is accessed (e.g. on the first query execution), or
• silently produces a dangling TypeRef for an undefined mutation / subscription root type, since schema { } root-type references were never existence-checked the way ordinary field/type references are.

Per the GraphQL spec, "Root Operation Types":

The query root operation type must be provided and must be an Object type. The mutation root operation type is optional [...] If it is provided, it must be an Object type. Similarly, the subscription root operation type is also optional [...] If it is provided, it must be an Object type.

This should be a schema validation error, not a runtime crash or silent bad state.

Fix

Adds SchemaValidator.validateRootTypes, wired into the existing validateSchema chain right after validateReferences. For each of query (mandatory), mutation (optional), subscription (optional):

• missing and mandatory → No 'query' root operation type in schema
• present but undefined → Undefined type '<name>' specified as '<op>' root operation type
• present, defined, but not an Object type → Type '<name>' specified as '<op>' root operation type is not an object type

Because both the schema"""...""" compile-time macro and the runtime Schema(string) constructor share the same SchemaParser → validateSchema pipeline, this closes the gap at construction time for both entry points. No changes needed to queryType/mutationType/subscriptionType themselves.

Test plan

• Three new targeted tests in SchemaSuite.scala, one per error path (missing query, dangling reference, non-object root type).
• ~12 pre-existing test fixtures that relied on Query being optional for schema construction (to unit-test unrelated validators in isolation) updated to include a minimal type Query { foo: Int }.