Implement session cache upgrade phase 1 foundation

MLXServerTests/Server/CancellationTokenTests.swift

@@ -0,0 +1,18 @@
+import XCTest
+@testable import MLX_Server
+
+final class CancellationTokenTests: XCTestCase {
+    func testStartsNotCancelled() {
+        let token = CancellationToken()
+
+        XCTAssertFalse(token.isCancelled)
+    }
+
+    func testCancelSetsFlag() {
+        let token = CancellationToken()
+
+        token.cancel()
+
+        XCTAssertTrue(token.isCancelled)
+    }
+}

MLXServerTests/Server/ImageDecoderTests.swift

@@ -0,0 +1,20 @@
+import XCTest
+@testable import MLX_Server
+
+final class ImageDecoderTests: XCTestCase {
+    private let onePixelPNGBase64 = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAFgwJ/lRyXWQAAAABJRU5ErkJggg=="
+
+    func testDecodeDataURI() {
+        let image = ImageDecoder.decode("data:image/png;base64,\(onePixelPNGBase64)")
+
+        XCTAssertNotNil(image)
+        XCTAssertGreaterThanOrEqual(image?.estimatedBytes ?? 0, 4)
+    }
+
+    func testDecodePlainBase64() {
+        let image = ImageDecoder.decode(onePixelPNGBase64)
+
+        XCTAssertNotNil(image)
+        XCTAssertGreaterThanOrEqual(image?.estimatedBytes ?? 0, 4)
+    }
+}

MLXServerTests/Server/StreamingSSEEncoderTests.swift

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+import XCTest
+@testable import MLX_Server
+
+final class StreamingSSEEncoderTests: XCTestCase {
+    func testEncodeContentDeltaMatchesJSONEncoderOutput() throws {
+        let encoder = StreamingSSEEncoder(requestId: "chatcmpl-test", created: 1_234_567, modelName: "qwen\"model")
+        let text = "line 1\nline 2\t\"quoted\"\\slash"
+
+        let actual = encoder.encodeContentDelta(text)
+        let expected = try baselineData(
+            for: APIChatCompletionChunk(
+                id: "chatcmpl-test",
+                object: "chat.completion.chunk",
+                created: 1_234_567,
+                model: "qwen\"model",
+                choices: [
+                    APIStreamChoice(
+                        index: 0,
+                        delta: APIDeltaMessage(role: nil, content: text, tool_calls: nil),
+                        finish_reason: nil
+                    )
+                ],
+                usage: nil
+            )
+        )
+
+        XCTAssertEqual(actual, expected)
+    }
+
+    func testEncodeRoleDeltaMatchesJSONEncoderOutput() throws {
+        let encoder = StreamingSSEEncoder(requestId: "chatcmpl-role", created: 99, modelName: "gemma")
+
+        let actual = encoder.encodeRoleDelta("assistant")
+        let expected = try baselineData(
+            for: APIChatCompletionChunk(
+                id: "chatcmpl-role",
+                object: "chat.completion.chunk",
+                created: 99,
+                model: "gemma",
+                choices: [
+                    APIStreamChoice(
+                        index: 0,
+                        delta: APIDeltaMessage(role: "assistant", content: nil, tool_calls: nil),
+                        finish_reason: nil
+                    )
+                ],
+                usage: nil
+            )
+        )
+
+        XCTAssertEqual(actual, expected)
+    }
+
+    func testEncodeFinalChunkMatchesBaseline() throws {
+        let chunk = APIChatCompletionChunk(
+            id: "chatcmpl-final",
+            object: "chat.completion.chunk",
+            created: 7,
+            model: "gemma",
+            choices: [
+                APIStreamChoice(
+                    index: 0,
+                    delta: APIDeltaMessage(role: nil, content: nil, tool_calls: nil),
+                    finish_reason: "stop"
+                )
+            ],
+            usage: APIUsageInfo(prompt_tokens: 10, completion_tokens: 3, total_tokens: 13)
+        )
+
+        XCTAssertEqual(StreamingSSEEncoder.encodeFinalChunk(chunk), try baselineData(for: chunk))
+    }
+
+    private func baselineData(for chunk: APIChatCompletionChunk) throws -> Data {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.sortedKeys]
+        let json = try encoder.encode(chunk)
+        var data = Data("data: ".utf8)
+        data.append(json)
+        data.append(Data("\n\n".utf8))
+        return data
+    }
+}