[Vertex AI] Add ImageGenerationInstance for input to predict call

FirebaseVertexAI/Sources/Types/Internal/Imagen/ImageGenerationInstance.swift

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+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+struct ImageGenerationInstance {
+  let prompt: String
+}
+
+// MARK: - Codable Conformance
+
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+extension ImageGenerationInstance: Encodable {}

FirebaseVertexAI/Tests/Unit/Types/Imagen/ImageGenerationInstanceTests.swift

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+// Copyright 2024 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import XCTest
+
+@testable import FirebaseVertexAI
+
+@available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
+final class ImageGenerationInstanceTests: XCTestCase {
+  let encoder = JSONEncoder()
+
+  override func setUp() {
+    encoder.outputFormatting = [.sortedKeys, .prettyPrinted]
+  }
+
+  // MARK: - Encoding Tests
+
+  func testEncodeInstance() throws {
+    let prompt = "An astronaut riding a horse."
+    let instance = ImageGenerationInstance(prompt: prompt)
+
+    let jsonData = try encoder.encode(instance)
+
+    let json = try XCTUnwrap(String(data: jsonData, encoding: .utf8))
+    XCTAssertEqual(json, """
+    {
+      "prompt" : "\(prompt)"
+    }
+    """)
+  }
+}