Adds Input Field Types mergeable validation rule

spec/Section 4 -- Composition.md

@@ -14,6 +14,82 @@ run in sequence to produce the composite execution schema.
 
 ### Pre Merge Validation
 
+#### Input Field Types mergeable
+
+**Error Code**
+
+`INPUT_FIELD_TYPES_NOT_MERGEABLE`
+
+**Formal Specification**
+
+- Let {fieldsByName} be a map of field lists where the key is the name of a
+  field and the value is a list of fields from mergeable input types from
+  different source schemas with the same name.
+- For each {fields} in {fieldsByName}:
+  - {InputFieldsAreMergeable(fields)} must be true.
+
+InputFieldsAreMergeable(fields):
+
+- Given each pair of members {fieldA} and {fieldB} in {fields}:
+  - Let {typeA} be the type of {fieldA}.
+  - Let {typeB} be the type of {fieldB}.
+  - {SameTypeShape(typeA, typeB)} must be true.
+
+**Explanatory Text**
+
+The input fields of input objects with the same name must be mergeable. This
+rule ensures that input objects with the same name in different source schemas
+have fields that can be merged consistently without conflicts.
+
+Input fields are considered mergeable when they share the same name and have
+compatible types. The compatibility of types is determined by their structure
+(e.g., lists), excluding nullability. Mergeable input fields with different
+nullability are considered mergeable, and the resulting merged field will be the
+most permissive of the two.
+
+In this example, the field `field` in `Input1` has compatible types across
+source schemas, making them mergeable:
+
+```graphql example
+input Input1 {
+  field: String!
+}
+
+input Input1 {
+  field: String
+}
+```
+
+The following example shows that fields are mergeable if they have different
+nullability but the named type is the same and the list structure is the same.
+
+```graphql example
+input Input1 {
+  tags: [String!]
+}
+
+input Input1 {
+  tags: [String]!
+}
+
+input Input1 {
+  tags: [String]
+}
+```
+
+In this example, the field `field` on `Input1` is not mergable as the field has
+different named types across (`String` and `DateTime`) source schemas:
+
+```graphql counter-example
+input Input1 {
+  field: String!
+}
+
+input Input1 {
+  field: DateTime!
+}
+```
+
 ### Merge
 
 ### Post Merge Validation