test: add regression test for promptTokens with strict firstTokenLogProbThreshold

Verifies that transcription produces non-empty output when promptTokens
are set, even with a strict firstTokenLogProbThreshold (-0.5). Before
the fix, the prompt-shifted first token logprob would trigger the
threshold and abort decoding immediately.

Author: alan890104

Tests/WhisperKitTests/UnitTests.swift

@@ -1702,6 +1702,34 @@ final class UnitTests: XCTestCase {
         XCTAssertFalse(result.text.contains(promptText), "Prompt text should not be present in the result")
     }
 
+    func testPromptTokensWithStrictFirstTokenThreshold() async throws {
+        // Regression test: promptTokens shift the decoder's KV cache state, which can
+        // cause the first content token's logprob to drop below firstTokenLogProbThreshold.
+        // With the fix, the threshold is skipped when promptTokens are present, so the
+        // decoder should still produce non-empty output even with a very strict threshold.
+        let config = WhisperKitConfig(model: "tiny", verbose: true, logLevel: .debug, load: true)
+        let whisperKit = try await WhisperKit(config)
+        let tokenizer = try XCTUnwrap(whisperKit.tokenizer)
+
+        let promptText = "繁體中文語音轉錄。"
+        let promptTokens = tokenizer.encode(text: promptText)
+
+        // Use a strict firstTokenLogProbThreshold that would have caused empty results
+        // before the fix (the prompt context shifts the first token's distribution).
+        let options = DecodingOptions(
+            skipSpecialTokens: true,
+            promptTokens: promptTokens,
+            firstTokenLogProbThreshold: -0.5
+        )
+
+        let result = try await XCTUnwrapAsync(
+            await transcribe(with: .tiny, options: options),
+            "Failed to transcribe"
+        )
+
+        XCTAssertFalse(result.text.isEmpty, "Transcription should not be empty when promptTokens are set, even with a strict firstTokenLogProbThreshold")
+    }
+
     func testPrefixTokens() async throws {
         let config = WhisperKitConfig(model: "tiny", verbose: true, logLevel: .debug, load: true)
         let whisperKit = try await WhisperKit(config)