Raise the LSP unknown-input-field diagnostic to Error severity

Diagnostics.descendUnknownArgs reports an input field that the SDL vocabulary does not declare at Warning severity. Every such field is a hard schema-load failure at build time (graphql-java rejects the directive application outright), so the editor tier is strictly softer than the build tier for a condition that cannot be ignored. An author sees a yellow squiggle, keeps typing, and the build refuses to load the schema.

Surfaced while reviewing the ExternalCodeReference.name removal. That change retired a bespoke Error-severity diagnostic at the name: coordinate and now leans on this generic validator, which reports Unknown field 'name' on input type 'ExternalCodeReference'. at Warning. The severity drop is not specific to that coordinate; it is the generic arm’s policy, and re-adding a per-coordinate arm is exactly the indirection that change removed. Fix it once, generically.

Scope is the severity on the descendUnknownArgs arm and the sibling validateUnknownArgs top-level "Unknown argument" arm, which has the same property. Check RejectionSeverityCoverageTest and the LSP severity conventions before flipping, and note that neither arm currently has any test pinning its message or severity, so the change wants a diagnostics-tier case per arm.