[nrf fromtree] storage/stream_flash: Add test for buffered data size API

Add a test for the new API to query the buffered data size in the
stream_flash subsystem.

Signed-off-by: Syver Haraldsen <[email protected]>
(cherry picked from commit bd9dc90)

Author: SyverHaraldsen

tests/subsys/storage/stream/stream_flash/src/main.c

@@ -276,6 +276,42 @@ ZTEST(lib_stream_flash, test_stream_flash_bytes_written)
 	VERIFY_WRITTEN(BUF_LEN, BUF_LEN);
 }
 
+ZTEST(lib_stream_flash, test_stream_flash_bytes_buffered)
+{
+	int rc;
+	size_t buffered;
+
+	init_target();
+
+	/* Initially no bytes should be buffered */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 0, "expected no buffered bytes");
+
+	/* Write partial buffer */
+	rc = stream_flash_buffered_write(&ctx, write_buf, BUF_LEN - 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* Verify buffered bytes */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, BUF_LEN - 128, "expected buffered bytes");
+
+	/* Write remaining buffer */
+	rc = stream_flash_buffered_write(&ctx, write_buf, 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* After auto-flush, no bytes should be buffered */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 0, "expected no buffered bytes");
+
+	/* Write more than buffer size to trigger auto-flush */
+	rc = stream_flash_buffered_write(&ctx, write_buf, BUF_LEN + 128, false);
+	zassert_equal(rc, 0, "expected success");
+
+	/* Verify buffered bytes */
+	buffered = stream_flash_bytes_buffered(&ctx);
+	zassert_equal(buffered, 128, "expected remaining buffered bytes after auto-flush");
+}
+
 ZTEST(lib_stream_flash, test_stream_flash_buf_size_greater_than_page_size)
 {
 	int rc;