Finish ignored validation conformance gaps from spec suite

[craigsmitham]

Summary

Issue #57 adds a new GraphZen.SpecConformance.Tests project for GraphQL draft Section 5 validation traceability. The suite is green locally, but only by explicitly ignoring the cases that currently expose implementation gaps or where the broader graphql-js matrix still needs to be ported.

This follow-up tracks the ignored validation conformance work so the suite can move from pass + ignore to fully enforced conformance.

Current state

• dotnet test passes on main with the new spec suite in place
• GraphZen.SpecConformance.Tests currently reports 91 passed, 21 skipped, 0 failed
• The skipped cases are intentional and correspond to product gaps or incomplete graphql-js parity ports

Ignored conformance areas

• 5.2.4.1 Single Root Field
  Negative subscription root-field validation cases are ignored
• 5.3.2 Field Selection Merging
  The broader graphql-js overlap matrix is not yet ported
• 5.3.3 Leaf Field Selections
  Negative validation cases are ignored
• 5.4.1 Argument Names
  Negative validation cases are ignored
• 5.4.2 Argument Uniqueness
  Negative validation cases are ignored
• 5.4.3 Required Arguments
  Negative validation cases are ignored
• 5.5.1.1 Fragment Name Uniqueness
  Negative validation cases are ignored
• 5.5.1.4 Fragments Must Be Used
  Negative validation cases are ignored
• 5.5.2.2 Fragment Spreads Must Not Form Cycles
  Negative validation cases are ignored
• 5.5.2.3 Fragment Spread Is Possible
  The broader graphql-js spread matrix is not yet ported
• 5.6.1, 5.6.2, 5.6.4
  The literal coercion / input field name / required field matrix is not yet ported
• 5.6.3 Input Object Field Uniqueness
  Negative validation cases are ignored
• 5.7.3 Directives Are Unique per Location
  Negative validation cases are ignored
• 5.8.1 Variable Uniqueness
  Negative validation case is ignored
• 5.8.2 Variables Are Input Types
  Negative validation case is ignored
• 5.8.3 All Variable Uses Defined
  Negative validation cases are ignored
• 5.8.4 All Variables Used
  Negative validation cases are ignored
• 5.8.5 All Variable Usages Are Allowed
  Negative validation cases are ignored, plus the OneOf-specific cases still need a dedicated schema harness

Acceptance criteria

• Replace ignored tests with enforced assertions where GraphZen already behaves correctly
• Fix implementation gaps uncovered by the ignored cases
• Port the remaining graphql-js validation matrices now represented only by placeholders
• Remove the temporary skips from GraphZen.SpecConformance.Tests
• Keep dotnet test green without relying on the ignored cases above