Validate required GraphQL variables at compile time

[velo]

Summary

• Adds compile-time validation that required GraphQL variables (non-null without default values) have corresponding Java method parameters
• Parameters annotated with @feign.Param are excluded from variable matching (they're header params)
• Errors point to the method element with actionable messages including variable name and type

Test plan

• missingRequiredVariableReportsError — missing required var emits error
• requiredVariableWithDefaultValueIsOptional — default value makes it optional
• nullableVariableIsOptional — nullable types don't require params
• paramAnnotationNotCountedAsVariable — @Param params excluded from matching
• requiredVariableWithMatchingParameterSucceeds — matching param passes
• multipleRequiredVariablesMissing — reports errors for each missing var
• All 26 tests pass (mvn test -pl graphql-apt)