Add unit tests

WooCommerce/src/test/kotlin/com/woocommerce/android/extensions/StringExtTest.kt

@@ -0,0 +1,57 @@
+package com.woocommerce.android.extensions
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+class StringExtTest {
+    @Test
+    fun `given string with valid size, when readableFileSize called, then returns formatted size`() {
+        // Given
+        val inputs = mapOf(
+            "1024" to "1 kB",
+            "1500000" to "1.4 MB",
+            "1234567" to "1.2 MB",
+            "1073741824" to "1 GB",
+            "1099511627776" to "1 TB"
+        )
+
+        // When & then
+        inputs.forEach { (input, expected) ->
+            assertThat(input.readableFileSize()).isEqualTo(expected)
+        }
+    }
+
+    @Test
+    fun `given zero or negative size, when readableFileSize called, then returns zero`() {
+        // Given
+        val inputs = listOf("0", "-1024", "-1")
+
+        // When & then
+        inputs.forEach { input ->
+            assertThat(input.readableFileSize()).isEqualTo("0")
+        }
+    }
+
+    @Test
+    fun `given invalid number string, when readableFileSize called, then returns zero`() {
+        // Given
+        val inputs = listOf("", "abc", "12.34", "1,000")
+
+        // When & then
+        inputs.forEach { input ->
+            assertThat(input.readableFileSize()).isEqualTo("0")
+        }
+    }
+
+    @Test
+    fun `given very large number, when readableFileSize called, then returns correct unit`() {
+        // Given
+        val size = "1" + "0".repeat(18) // 1 quintillion bytes
+
+        // When
+        val result = size.readableFileSize()
+
+        // Then
+        assertThat(result).isEqualTo("888.2 PB")
+    }
+}