Assign to properties with explicit self in init(from decoder:) (#696)

### Motivation

Our generated `init(from decoder:)` assigns to properties without using
explicit self. This is a problem because it can produce conflicts with
the local variables created in the initializer, e.g. the decoding
container, which will then fail to compile for schemas with properties
named `container`, as found by an adopter in #695.

We should probably take a broader pass with regard to the use of
explicit self in our generated code, but for now we can resolve this by
focussing on this problematic generation, where a schema with named and
additional properties.

### Modifications

- Assign to properties with explicit self in `init(from decoder:)`

### Result

- Fixes #695.

### Test Plan

- Add a snippet test.
- Update the reference tests.

Author: simonjbeaumont

Sources/_OpenAPIGeneratorCore/Translator/CommonTranslations/translateCodable.swift

@@ -118,7 +118,7 @@ extension FileTranslator {
         let assignExprs: [Expression] = properties.map { property in
             let typeUsage = property.typeUsage
             return .assignment(
-                left: .identifierPattern(property.swiftSafeName),
+                left: .identifierPattern("self").dot(property.swiftSafeName),
                 right: .try(
                     .identifierPattern("container").dot("decode\(typeUsage.isOptional ? "IfPresent" : "")")
                         .call([

Tests/OpenAPIGeneratorReferenceTests/Resources/ReferenceSources/Petstore/Types.swift

@@ -594,7 +594,7 @@ public enum Components {
             }
             public init(from decoder: any Decoder) throws {
                 let container = try decoder.container(keyedBy: CodingKeys.self)
-                foo = try container.decodeIfPresent(
+                self.foo = try container.decodeIfPresent(
                     Swift.String.self,
                     forKey: .foo
                 )
@@ -626,7 +626,7 @@ public enum Components {
             }
             public init(from decoder: any Decoder) throws {
                 let container = try decoder.container(keyedBy: CodingKeys.self)
-                foo = try container.decodeIfPresent(
+                self.foo = try container.decodeIfPresent(
                     Swift.String.self,
                     forKey: .foo
                 )
@@ -666,7 +666,7 @@ public enum Components {
             }
             public init(from decoder: any Decoder) throws {
                 let container = try decoder.container(keyedBy: CodingKeys.self)
-                foo = try container.decodeIfPresent(
+                self.foo = try container.decodeIfPresent(
                     Swift.String.self,
                     forKey: .foo
                 )

Tests/OpenAPIGeneratorReferenceTests/SnippetBasedReferenceTests.swift

@@ -510,15 +510,30 @@ final class SnippetBasedReferenceTests: XCTestCase {
             schemas:
               MyObject:
                 type: object
-                properties: {}
+                properties:
+                  id:
+                    type: string
                 additionalProperties: false
             """,
             """
             public enum Schemas {
                 public struct MyObject: Codable, Hashable, Sendable {
-                    public init() {}
+                    public var id: Swift.String?
+                    public init(id: Swift.String? = nil) {
+                        self.id = id
+                    }
+                    public enum CodingKeys: String, CodingKey {
+                        case id
+                    }
                     public init(from decoder: any Decoder) throws {
-                        try decoder.ensureNoAdditionalProperties(knownKeys: [])
+                        let container = try decoder.container(keyedBy: CodingKeys.self)
+                        self.id = try container.decodeIfPresent(
+                            Swift.String.self,
+                            forKey: .id
+                        )
+                        try decoder.ensureNoAdditionalProperties(knownKeys: [
+                            "id"
+                        ])
                     }
                 }
             }