feat(readarr): test an indexer and expose the command on the CLI

Fetch the indexer's full details, then post them back verbatim to the
test endpoint. The payload round-trips through serde_json::Value so the
fields Indexer does not model, such as infoLink and the per-field label
and type metadata, survive the round trip intact.

A successful test returns an empty JSON object and clears
indexer_test_errors; a failure returns an array whose first entry
carries the error message. The test endpoint reports failures with a
non-2xx status, so the request ignores the status code and parses the
body either way.

src/cli/readarr/mod.rs

@@ -110,6 +110,13 @@ pub enum ReadarrCommand {
     )]
     task_name: ReadarrTaskName,
   },
+  #[command(
+    about = "Test the indexer with the given ID. Note that a successful test returns an empty JSON body; i.e. '{}'"
+  )]
+  TestIndexer {
+    #[arg(long, help = "The ID of the indexer to test", required = true)]
+    indexer_id: i64,
+  },
   #[command(
     about = "Toggle monitoring for the specified author corresponding to the given author ID"
   )]
@@ -240,6 +247,13 @@ impl<'a, 'b> CliCommandHandler<'a, 'b, ReadarrCommand> for ReadarrCliHandler<'a,
           .await?;
         serde_json::to_string_pretty(&resp)?
       }
+      ReadarrCommand::TestIndexer { indexer_id } => {
+        let resp = self
+          .network
+          .handle_network_event(ReadarrEvent::TestIndexer(indexer_id).into())
+          .await?;
+        serde_json::to_string_pretty(&resp)?
+      }
       ReadarrCommand::ToggleAuthorMonitoring { author_id } => {
         let resp = self
           .network