[C# operations] - generates duplicate classes

**Describe the bug**
When more properties of the same type are included in the same query/mutation, generator outputs duplicated classes with the same name in result cs file

**To Reproduce**
1. Include more fields with the same non-primitive type but different name in the schema
2. Make query/mutation to return all of the fields with the same type
3. Generate result cs file
Using example data in this issue, it generates TestSelection class 2-times in the same subclass which results to syntax error in C#.

1. My GraphQL schema:

```graphql
scalar Date

schema {
query: Query
}

type Query {
me: User!
another: User!
user(id: ID!): User
allUsers: [User]
search(term: String!): [SearchResult!]!
myChats: [Chat!]!
}

enum Role {
USER,
ADMIN,
}

interface Node {
id: ID!
}

union SearchResult = User | Chat | ChatMessage

type User implements Node {
id: ID!
username: String!
email: String!
role: Role!
test1: Test!
test2: Test!
}

type Test{
id: ID!
message: String!
}

type Chat implements Node {
id: ID!
users: [User!]!
messages: [ChatMessage!]!
}

type ChatMessage implements Node {
id: ID!
content: String!
time: Date!
fromUser: User!
toUser: User!
}

```

2. My GraphQL operations:

```graphql
query findUser($userId: ID!) {
user(id: $userId) {
...UserFields
}
}

fragment UserFields on User {
id
username
role
test1{
id
}
test2{
id
}
}
```

3. My `codegen.yml` config file:

```yml
generates:
src/main/c-sharp/my-org/my-app/Operations.cs:
plugins:
- c-sharp-operations
config:
typesafeOperation: true
```

**Expected behavior**
"Selection" classes should be named by fieldName instead of fieldType =>
1. public class Test1Selection
2. public class Test2Selection

**Environment:**
- `@graphql-codegen/c-sharp-operations`:

Start at the c-sharp-operations plugin entry point and reproduce the issue using the schema, UserFields operation, and codegen.yml shown here. Inspect the generated src/main/c-sharp/my-org/my-app/Operations.cs and verify that the result contains distinct Test1Selection and Test2Selection classes without a C# syntax error.

Written by the indexing model from the issue text.