[joadan]

Third slice of the agent package described in docs/design/agent-mode.md: every query the agent writes is checked against the scoped schema before it is sent.

Changes

Validator (QueryValidator, internal)

• Document: exactly one operation, which must be a query; no schema definitions; clear syntax errors.
• Selections: only allowed roots; fields that exist and aren't omitted; no selection on leaf fields and a required selection on object fields; aliases ignored, since the check uses real field names; fragments and inline fragments followed through their type conditions, including interfaces, with undefined and self-spreading fragments caught.
• Arguments and values: unknown and missing required arguments, filter objects field by field, enum values, scalar kinds, null into non-null.
• Variables: declared, with a type that is in scope and matches where it is used. Variable values are checked too, so an omitted field cannot be passed in through a variable.
• Blocked: introspection (__schema, __type) and directives other than @include and @skip.
• All errors are returned at once, each with a path and a fix: available names, "did you mean" suggestions, the paging pattern for paged fields, example selections, and enum values without quotes.

Design doc: slice status, and the note that variable values are validated.

Testing

• 267 tests pass, 26 new (QueryValidatorTests):
  • Every query the validator accepts is also run against the in-process test server and must succeed, so the validator cannot be more lenient than the server.
  • An omitted field cannot be reached by selecting it, filtering on it as a literal, filtering on it through a variable, or sorting on it.
  • Variables, arguments, enums, fragments, document rules, and all errors reported at once.