fix(setup): configure an empty Hermes config.yaml (#1983)

* fix(setup): treat an empty Hermes config.yaml as a new document

YAML parses an empty file to null, so Hermes configuration was checked
as an object before the shared empty-file rule and rejected. Setup then
failed its readback gate for a zero-byte config.yaml, although a missing
file is configured and the Hermes serializer already writes a fresh
document for empty original text. Apply the empty-file rule before the
Hermes YAML parse, as #1117 did for JSON clients.

* test(setup): configure an empty Hermes config.yaml

Configure zero-byte and newline-only config.yaml files through the
real filesystem workflow, then read the registration back as aligned.

src/application/ClientConfigurationDocument.ts

@@ -104,11 +104,11 @@ const parseDocument = (
   // comments, trailing commas, a BOM, or an empty existing file must be
   // rejected instead of registering into a document Pi cannot load.
   if (format === "pi") return objectSchema.parse(JSON.parse(text));
-  // Hermes stores MCP servers in `mcp_servers` inside a YAML document.
-  if (format === "hermes") return objectSchema.parse(parseYaml(text));
   // An empty file, which some clients create before any server is added,
   // holds no settings to preserve.
   if (isEmptyClientConfigurationText(text)) return {};
+  // Hermes stores MCP servers in `mcp_servers` inside a YAML document.
+  if (format === "hermes") return objectSchema.parse(parseYaml(text));
   const errors: ParseError[] = [];
   // Accept a UTF-8 BOM without shifting diagnostics or editing the original text.
   const jsonText = text.startsWith("\uFEFF") ? ` ${text.slice(1)}` : text;

tests/boundary/filesystem/hermesClientConfiguration.test.ts

@@ -185,4 +185,28 @@ mcp_servers:
       },
     });
   });
+
+  it.each(["", "\n"])(
+    "configures an empty config.yaml %j as a new document",
+    async (original) => {
+      const { home, client } = await hermesClient();
+      await writeFile(client.configPath, original);
+      expect(
+        (await configureClientConfiguration(client, {}, command)).status,
+      ).toBe("configured");
+      expect(parseYaml(await readFile(client.configPath, "utf8"))).toEqual({
+        mcp_servers: {
+          rea: { command: command[0], args: command.slice(1), enabled: true },
+        },
+      });
+      expect(
+        (
+          await readClientRegistrationStatuses(home, undefined, {
+            platform: process.platform,
+            environment: {},
+          })
+        ).find(({ client: name }) => name === "hermes"),
+      ).toMatchObject({ state: "aligned", command });
+    },
+  );
 });