Fix type validator inconsistencies and improve variable error reporting

[curtisman]

Changes

• Fix + operator pass1/pass2 consistency: Changed || to && in the + case of deriveValueTypeImpl so pass 1 type inference matches pass 2 validation semantics — both operands must be strings to infer string.

• Use formatSchemaType() in error messages: Replaced raw .type discriminant strings with formatSchemaType() for richer, human-readable type descriptions (e.g. string | number instead of type-union). Removed the redundant union-iteration branch in validateValueType that duplicated logic already handled by isTypeAssignable.

• Pre-resolve expected type in validateVariableAgainstSchema: Resolve the expected type and short-circuit on any before calling into structural validation.

• Add symmetric coinductive guard for type-reference cycles: The isTypeAssignable visited-set now guards both inferred and expected type-references, preventing infinite recursion when the expected type is self-recursive (e.g. A = string | A).

• Replace shallow isTypeAssignable with structural validateInferredAgainstExpected: Both validateVariableType and validateVariableAgainstSchema now use structural validation so object fields and array element types are checked, not just the top-level type discriminant.

• Remove dead resolvedTypes parameter: Eliminated the unused resolvedTypes: Map<string, SchemaTypeDefinition> parameter from validateValueType, validateObjectValue, validateArrayValue, and validateVariableAgainstSchema.

• Include variable name in validation error messages: Errors now report which variable caused the mismatch (e.g. Variable '$myVar' expected number, got string).

• Add tests for variable name error reporting: Three new test cases verify that variable names appear in error messages for direct type mismatches, implicit variable mismatches, and nested field errors.

• Add TODO for boolean literal type precision: Documented a future improvement to return true/false literal types from boolean constants (blocked on actionSchema lacking boolean literal factories).