Replace force-unwrapped Range lookups with XCTUnwrap in ExporterTests

Addresses a Copilot review on #202. A missing key in the palette JSON now
reports a clean test failure instead of crashing the XCTest runner.

https://claude.ai/code/session_01JcvsVeuayd5NE72hbntxbK

Author: claude

PikaTests/ExporterTests.swift

@@ -87,16 +87,17 @@ final class ExporterTests: XCTestCase {
         XCTAssertTrue(colors.isEmpty)
     }
 
-    func test_paletteToJSON_outputIsPrettyPrintedAndSorted() {
+    func test_paletteToJSON_outputIsPrettyPrintedAndSorted() throws {
         // The implementation asks JSONSerialization for both `.prettyPrinted`
         // and `.sortedKeys`. Preserve that contract so CLI diffs of exported
         // palettes stay stable.
         let pair = makePair(fg: "#ff0000", bg: "#00ff00", date: Date(timeIntervalSince1970: 0))
         let json = Exporter.paletteToJSON(pairs: [pair], name: "Brand")
         XCTAssertTrue(json.contains("\n"), "Output should be pretty-printed across multiple lines")
         // `colors` sorts alphabetically before `name` when sortedKeys is on.
-        let colorsIndex = json.range(of: "\"colors\"")!.lowerBound
-        let nameIndex = json.range(of: "\"name\"")!.lowerBound
-        XCTAssertLessThan(colorsIndex, nameIndex, "Keys should be alphabetically sorted")
+        let colorsRange = try XCTUnwrap(json.range(of: "\"colors\""))
+        let nameRange = try XCTUnwrap(json.range(of: "\"name\""))
+        XCTAssertLessThan(colorsRange.lowerBound, nameRange.lowerBound,
+                          "Keys should be alphabetically sorted")
     }
 }