[3/n] Add an MCP stdio example (#337)

### Summary:
Spins up a stdio server with some local files, then asks the model
questions.

### Test Plan:
Run the example, see it work.

(repeat of #322)

examples/mcp/filesystem_example/README.md

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+# MCP Filesystem Example
+
+This example uses the [fileystem MCP server](https://github.com/modelcontextprotocol/servers/tree/main/src/filesystem), running locally via `npx`.
+
+Run it via:
+
+```
+uv run python  python examples/mcp/filesystem_example/main.py
+```
+
+## Details
+
+The example uses the `MCPServerStdio` class from `agents`, with the command:
+
+```bash
+npx -y "@modelcontextprotocol/server-filesystem" <samples_directory>
+```
+
+It's only given access to the `sample_files` directory adjacent to the example, which contains some sample data.
+
+Under the hood:
+
+1. The server is spun up in a subprocess, and exposes a bunch of tools like `list_directory()`, `read_file()`, etc.
+2. We add the server instance to the Agent via `mcp_agents`.
+3. Each time the agent runs, we call out to the MCP server to fetch the list of tools via `server.list_tools()`.
+4. If the LLM chooses to use an MCP tool, we call the MCP server to run the tool via `server.run_tool()`.

examples/mcp/filesystem_example/main.py

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+import asyncio
+import os
+import shutil
+
+from agents import Agent, Runner, trace
+from agents.mcp import MCPServer, MCPServerStdio
+
+
+async def run(mcp_server: MCPServer):
+    agent = Agent(
+        name="Assistant",
+        instructions="Use the tools to read the filesystem and answer questions based on those files.",
+        mcp_servers=[mcp_server],
+    )
+
+    # List the files it can read
+    message = "Read the files and list them."
+    print(f"Running: {message}")
+    result = await Runner.run(starting_agent=agent, input=message)
+    print(result.final_output)
+
+    # Ask about books
+    message = "What is my #1 favorite book?"
+    print(f"\n\nRunning: {message}")
+    result = await Runner.run(starting_agent=agent, input=message)
+    print(result.final_output)
+
+    # Ask a question that reads then reasons.
+    message = "Look at my favorite songs. Suggest one new song that I might like."
+    print(f"\n\nRunning: {message}")
+    result = await Runner.run(starting_agent=agent, input=message)
+    print(result.final_output)
+
+
+async def main():
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    samples_dir = os.path.join(current_dir, "sample_files")
+
+    async with MCPServerStdio(
+        params={
+            "command": "npx",
+            "args": ["-y", "@modelcontextprotocol/server-filesystem", samples_dir],
+        }
+    ) as server:
+        with trace(workflow_name="MCP Filesystem Example"):
+            await run(server)
+
+
+if __name__ == "__main__":
+    # Let's make sure the user has npx installed
+    if not shutil.which("npx"):
+        raise RuntimeError("npx is not installed. Please install it with `npm install -g npx`.")
+
+    asyncio.run(main())

examples/mcp/filesystem_example/sample_files/favorite_books.txt

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+1. To Kill a Mockingbird – Harper Lee
+2. Pride and Prejudice – Jane Austen
+3. 1984 – George Orwell
+4. The Hobbit – J.R.R. Tolkien
+5. Harry Potter and the Sorcerer’s Stone – J.K. Rowling
+6. The Great Gatsby – F. Scott Fitzgerald
+7. Charlotte’s Web – E.B. White
+8. Anne of Green Gables – Lucy Maud Montgomery
+9. The Alchemist – Paulo Coelho
+10. Little Women – Louisa May Alcott
+11. The Catcher in the Rye – J.D. Salinger
+12. Animal Farm – George Orwell
+13. The Chronicles of Narnia: The Lion, the Witch, and the Wardrobe – C.S. Lewis
+14. The Book Thief – Markus Zusak
+15. A Wrinkle in Time – Madeleine L’Engle
+16. The Secret Garden – Frances Hodgson Burnett
+17. Moby-Dick – Herman Melville
+18. Fahrenheit 451 – Ray Bradbury
+19. Jane Eyre – Charlotte Brontë
+20. The Little Prince – Antoine de Saint-Exupéry

examples/mcp/filesystem_example/sample_files/favorite_cities.txt

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+- In the summer, I love visiting London.
+- In the winter, Tokyo is great.
+- In the spring, San Francisco.
+- In the fall, New York is the best.

examples/mcp/filesystem_example/sample_files/favorite_songs.txt

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+1. "Here Comes the Sun" – The Beatles
+2. "Imagine" – John Lennon
+3. "Bohemian Rhapsody" – Queen
+4. "Shake It Off" – Taylor Swift
+5. "Billie Jean" – Michael Jackson
+6. "Uptown Funk" – Mark Ronson ft.  Bruno Mars
+7. "Don’t Stop Believin’" – Journey
+8. "Dancing Queen" – ABBA
+9. "Happy" – Pharrell Williams
+10. "Wonderwall" – Oasis