feat: Add @semanticNonNull directive support

[jwaldrip]

Summary

Implements the @semanticNonNull directive as proposed by the GraphQL nullability working group. This directive allows schema authors to indicate that a field is semantically non-null (the resolver never intentionally returns null), but may still be null due to errors.

This decouples nullability from error handling, providing better developer experience for clients who want to understand which fields may be null only due to errors versus fields that may intentionally be null.

Changes

• Add @semanticNonNull directive to prototype notation with levels argument
• Create Absinthe.Type.SemanticNullability support module with helper functions
• Add isSemanticNonNull and semanticNonNullLevels to __Field introspection
• Add semantic_non_null: true/[levels] shorthand for field definitions

Usage

# Using directive syntax
field :email, :string do
  directive :semantic_non_null
end

# Using shorthand notation
field :email, :string, semantic_non_null: true
field :posts, list_of(:post), semantic_non_null: [0, 1]

Test plan

• Added comprehensive tests (31 new tests)
• All 1473 tests pass
• Directive definition and configuration tested
• Introspection queries return correct values
• Shorthand notation works correctly

🤖 Generated with Claude Code

[cschiewek]

Personally, I'd prefer not to add this until it hits the actual working draft of the spec, but given it's opt-in, if the majority of @absinthe-graphql/core are fine with it, I won't stop it.