test: add MCP client e2e tests with real MCP servers

Prove the full MCP protocol flow works end-to-end against real servers:
- Self-written fixture server: tool discovery, execution, error handling,
  image placeholder, toolPrefix, and clean disposal
- @modelcontextprotocol/server-everything: echo, get-sum, get-tiny-image
- @modelcontextprotocol/server-filesystem: write_file + read_file round-trip,
  list_directory with world-verification

All 15 tests keyless and deterministic (no API key needed).

packages/mcp/mcp-client/tests/fixture-server.ts

@@ -0,0 +1,55 @@
+/**
+ * Minimal MCP server over stdio for e2e testing of the dsh-mcp-client plugin.
+ * Registers controlled tools with predictable behavior for asserting edge cases.
+ *
+ * Run: node --import tsx fixture-server.ts
+ */
+
+import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js'
+import { StdioServerTransport } from '@modelcontextprotocol/sdk/server/stdio.js'
+import { z } from 'zod'
+
+const server = new McpServer(
+  { name: 'fixture-server', version: '1.0.0' },
+  { capabilities: { tools: { listChanged: true } } },
+)
+
+server.registerTool('add', {
+  title: 'Add Tool',
+  description: 'Adds two numbers.',
+  inputSchema: { a: z.number().describe('First number'), b: z.number().describe('Second number') },
+}, async args => ({
+  content: [{ type: 'text', text: String(args.a + args.b) }],
+}))
+
+server.registerTool('greet', {
+  title: 'Greet Tool',
+  description: 'Greets a person by name.',
+  inputSchema: { name: z.string().describe('Name to greet') },
+}, async args => ({
+  content: [{ type: 'text', text: `Hello, ${args.name}!` }],
+}))
+
+server.registerTool('fail', {
+  title: 'Fail Tool',
+  description: 'Always returns an error.',
+  inputSchema: {},
+}, async () => ({
+  content: [{ type: 'text', text: 'Something went wrong' }],
+  isError: true,
+}))
+
+server.registerTool('image', {
+  title: 'Image Tool',
+  description: 'Returns an image content block.',
+  inputSchema: {},
+}, async () => ({
+  content: [
+    { type: 'text', text: 'Here is an image:' },
+    { type: 'image', data: 'iVBORw0KGgo=', mimeType: 'image/png' },
+    { type: 'text', text: 'End of image.' },
+  ],
+}))
+
+const transport = new StdioServerTransport()
+await server.connect(transport)