Assert that a negative extractor sniff never advances the read position

Extractor#sniff() javadoc says:
"If true is returned, the input's reading position may have been
modified. Otherwise, only its peek position may have been modified."
https://exoplayer.dev/doc/reference/com/google/android/exoplayer2/extractor/Extractor.html#sniff-com.google.android.exoplayer2.extractor.ExtractorInput-

PiperOrigin-RevId: 314296922

library/core/src/main/java/com/google/android/exoplayer2/source/BundledExtractorsAdapter.java

@@ -54,7 +54,8 @@ import java.io.IOException;
   public void init(
       DataReader dataReader, Uri uri, long position, long length, ExtractorOutput output)
       throws IOException {
-    extractorInput = new DefaultExtractorInput(dataReader, position, length);
+    ExtractorInput extractorInput = new DefaultExtractorInput(dataReader, position, length);
+    this.extractorInput = extractorInput;
     if (extractor != null) {
       return;
     }
@@ -70,6 +71,7 @@ import java.io.IOException;
         } catch (EOFException e) {
           // Do nothing.
         } finally {
+          Assertions.checkState(this.extractor != null || extractorInput.getPosition() == position);
           extractorInput.resetPeekPosition();
         }
       }

testutils/src/main/java/com/google/android/exoplayer2/testutil/ExtractorAsserts.java

@@ -155,9 +155,13 @@ public final class ExtractorAsserts {
    */
   public static void assertSniff(
       Extractor extractor, FakeExtractorInput input, boolean expectedResult) throws IOException {
+    long originalPosition = input.getPosition();
     while (true) {
       try {
         assertThat(extractor.sniff(input)).isEqualTo(expectedResult);
+        if (!expectedResult) {
+          assertThat(input.getPosition()).isEqualTo(originalPosition);
+        }
         return;
       } catch (SimulatedIOException e) {
         // Ignore.