Fixed ThunderbirdAutoconfigFetcher throwing on IOExceptions

If the requested url does not exist then the `fetchAutoconfigFile` would just crash instead of returning `null`. This would mean that the discovery code would also crash instead of trying other URLs.

app/autodiscovery/thunderbird/build.gradle.kts

@@ -11,4 +11,5 @@ dependencies {
     implementation(libs.okhttp)
 
     testImplementation(libs.kxml2)
+    testImplementation(libs.okhttp.mockwebserver)
 }

app/autodiscovery/thunderbird/src/main/java/com/fsck/k9/autodiscovery/thunderbird/ThunderbirdAutoconfigFetcher.kt

@@ -1,5 +1,7 @@
 package com.fsck.k9.autodiscovery.thunderbird
 
+import com.fsck.k9.logging.Timber
+import java.io.IOException
 import java.io.InputStream
 import okhttp3.HttpUrl
 import okhttp3.OkHttpClient
@@ -8,13 +10,18 @@ import okhttp3.Request
 class ThunderbirdAutoconfigFetcher(private val okHttpClient: OkHttpClient) {
 
     fun fetchAutoconfigFile(url: HttpUrl): InputStream? {
-        val request = Request.Builder().url(url).build()
+        return try {
+            val request = Request.Builder().url(url).build()
 
-        val response = okHttpClient.newCall(request).execute()
+            val response = okHttpClient.newCall(request).execute()
 
-        return if (response.isSuccessful) {
-            response.body?.byteStream()
-        } else {
+            if (response.isSuccessful) {
+                response.body?.byteStream()
+            } else {
+                null
+            }
+        } catch (e: IOException) {
+            Timber.d(e, "Error fetching URL: %s", url)
             null
         }
     }

app/autodiscovery/thunderbird/src/test/java/com/fsck/k9/autodiscovery/thunderbird/ThunderbirdAutoconfigFetcherTest.kt

@@ -0,0 +1,44 @@
+package com.fsck.k9.autodiscovery.thunderbird
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import assertk.assertions.isNull
+import kotlin.test.assertNotNull
+import okhttp3.HttpUrl.Companion.toHttpUrl
+import okhttp3.OkHttpClient
+import okhttp3.mockwebserver.MockResponse
+import okhttp3.mockwebserver.MockWebServer
+import org.junit.Test
+
+class ThunderbirdAutoconfigFetcherTest {
+    private val fetcher = ThunderbirdAutoconfigFetcher(OkHttpClient.Builder().build())
+
+    @Test
+    fun shouldHandleNonexistentUrl() {
+        val nonExistentUrl =
+            "https://autoconfig.domain.invalid/mail/config-v1.1.xml?emailaddress=test%40domain.example".toHttpUrl()
+
+        val inputStream = fetcher.fetchAutoconfigFile(nonExistentUrl)
+
+        assertThat(inputStream).isNull()
+    }
+
+    @Test
+    fun shouldHandleEmptyResponse() {
+        val server = MockWebServer().apply {
+            this.enqueue(
+                MockResponse()
+                    .setBody("")
+                    .setResponseCode(204),
+            )
+            start()
+        }
+        val url = server.url("/empty/")
+
+        val inputStream = fetcher.fetchAutoconfigFile(url)
+
+        assertNotNull(inputStream) { inputStream ->
+            assertThat(inputStream.available()).isEqualTo(0)
+        }
+    }
+}