feat(app): throw helpful error when callServerTool is called with a string (#449)

Users frequently call callServerTool("tool_name", args) instead of the
correct callServerTool({ name: "tool_name", arguments: args }), resulting
in a silent/confusing failure. Detect this and throw immediately with
a message that shows the correct call shape.

Also fixes the incorrect example in examples/pdf-server/README.md that
demonstrated this same wrong usage pattern.

Fixes #386

https://claude.ai/code/session_01GqAyN4Ux7svWoU2HqsSLZF

Co-authored-by: Claude <noreply@anthropic.com>

src/app.ts

@@ -724,6 +724,12 @@ export class App extends Protocol<AppRequest, AppNotification, AppResult> {
     params: CallToolRequest["params"],
     options?: RequestOptions,
   ): Promise<CallToolResult> {
+    if (typeof params === "string") {
+      throw new Error(
+        `callServerTool() expects an object as its first argument, but received a string ("${params}"). ` +
+          `Did you mean: callServerTool({ name: "${params}", arguments: { ... } })?`,
+      );
+    }
     return await this.request(
       { method: "tools/call", params },
       CallToolResultSchema,