fix: show custom error for modal state (#240)

Calling a tool that resolves modal state, when there's no such modal
state visible, currently shows this misleading message:

```md
Tool "browser_file_upload" does not handle the modal state.
### Modal state
```

Instead, we should show the error message from the tool implementation.

src/context.ts

@@ -58,6 +58,8 @@ export class Context {
 
   modalStatesMarkdown(): string[] {
     const result: string[] = ['### Modal state'];
+    if (this._modalStates.length === 0)
+      result.push('- There is no modal state present');
     for (const state of this._modalStates) {
       const tool = this.tools.find(tool => tool.clearsModalState === state.type);
       result.push(`- [${state.description}]: can be handled by the "${tool?.schema.name}" tool`);

src/server.ts

@@ -49,34 +49,25 @@ export function createServerWithTools(serverOptions: MCPServerOptions, config: C
   });
 
   server.setRequestHandler(CallToolRequestSchema, async request => {
+    const errorResult = (...messages: string[]) => ({
+      content: [{ type: 'text', text: messages.join('\n') }],
+      isError: true,
+    });
     const tool = tools.find(tool => tool.schema.name === request.params.name);
-    if (!tool) {
-      return {
-        content: [{ type: 'text', text: `Tool "${request.params.name}" not found` }],
-        isError: true,
-      };
-    }
+    if (!tool)
+      return errorResult(`Tool "${request.params.name}" not found`);
+
 
     const modalStates = context.modalStates().map(state => state.type);
-    if ((tool.clearsModalState && !modalStates.includes(tool.clearsModalState)) ||
-        (!tool.clearsModalState && modalStates.length)) {
-      const text = [
-        `Tool "${request.params.name}" does not handle the modal state.`,
-        ...context.modalStatesMarkdown(),
-      ].join('\n');
-      return {
-        content: [{ type: 'text', text }],
-        isError: true,
-      };
-    }
+    if (tool.clearsModalState && !modalStates.includes(tool.clearsModalState))
+      return errorResult(`The tool "${request.params.name}" can only be used when there is related modal state present.`, ...context.modalStatesMarkdown());
+    if (!tool.clearsModalState && modalStates.length)
+      return errorResult(`Tool "${request.params.name}" does not handle the modal state.`, ...context.modalStatesMarkdown());
 
     try {
       return await context.run(tool, request.params.arguments);
     } catch (error) {
-      return {
-        content: [{ type: 'text', text: String(error) }],
-        isError: true,
-      };
+      return errorResult(String(error));
     }
   });
 

tests/files.spec.ts

@@ -30,6 +30,17 @@ test('browser_file_upload', async ({ client }) => {
   - button "Button" [ref=s1e4]
 \`\`\``);
 
+  {
+    expect(await client.callTool({
+      name: 'browser_file_upload',
+      arguments: { paths: [] },
+    })).toHaveTextContent(`
+The tool "browser_file_upload" can only be used when there is related modal state present.
+### Modal state
+- There is no modal state present
+      `.trim());
+  }
+
   expect(await client.callTool({
     name: 'browser_click',
     arguments: {