Validate SDL directive argument values

[afurm]

Summary

• Add SDL validation for directive argument value types, including custom directive usages.
• Reuse ValuesOfCorrectTypeRule for SDL directive nodes through a directive-aware validation context.
• Add coverage for validateSDL and buildSchema rejecting invalid directive argument values.

Why

Fixes #3912.

SDL already rejects invalid values for built-in directives, but custom directive usages could accept invalid argument values such as @foo(bar: FOOBAR) when bar is declared as String!.

Validation

• npm run lint
• npm run check
• npm run testonly
• npm run prettier:check
• git diff --check

Full npm test was attempted, but the Docker-backed supported-node integration could not run locally because the Docker daemon was unavailable.

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