Integrate FastMCP

This commit integrates FastMCP, a high-level MCP server implementation originally written by Jeremiah Lowin,
into the official MCP SDK. It also updates dependencies and adds new dev dependencies.
It moves the existing SDK into a .lowlevel .

tests/server/fastmcp/prompts/test_base.py

@@ -0,0 +1,194 @@
+from pydantic import FileUrl
+import pytest
+from mcp.server.fastmcp.prompts.base import (
+    Prompt,
+    UserMessage,
+    TextContent,
+    AssistantMessage,
+    Message,
+)
+from mcp.types import EmbeddedResource, TextResourceContents
+
+
+class TestRenderPrompt:
+    async def test_basic_fn(self):
+        def fn() -> str:
+            return "Hello, world!"
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(content=TextContent(type="text", text="Hello, world!"))
+        ]
+
+    async def test_async_fn(self):
+        async def fn() -> str:
+            return "Hello, world!"
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(content=TextContent(type="text", text="Hello, world!"))
+        ]
+
+    async def test_fn_with_args(self):
+        async def fn(name: str, age: int = 30) -> str:
+            return f"Hello, {name}! You're {age} years old."
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render(arguments=dict(name="World")) == [
+            UserMessage(
+                content=TextContent(
+                    type="text", text="Hello, World! You're 30 years old."
+                )
+            )
+        ]
+
+    async def test_fn_with_invalid_kwargs(self):
+        async def fn(name: str, age: int = 30) -> str:
+            return f"Hello, {name}! You're {age} years old."
+
+        prompt = Prompt.from_function(fn)
+        with pytest.raises(ValueError):
+            await prompt.render(arguments=dict(age=40))
+
+    async def test_fn_returns_message(self):
+        async def fn() -> UserMessage:
+            return UserMessage(content="Hello, world!")
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(content=TextContent(type="text", text="Hello, world!"))
+        ]
+
+    async def test_fn_returns_assistant_message(self):
+        async def fn() -> AssistantMessage:
+            return AssistantMessage(
+                content=TextContent(type="text", text="Hello, world!")
+            )
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            AssistantMessage(content=TextContent(type="text", text="Hello, world!"))
+        ]
+
+    async def test_fn_returns_multiple_messages(self):
+        expected = [
+            UserMessage("Hello, world!"),
+            AssistantMessage("How can I help you today?"),
+            UserMessage("I'm looking for a restaurant in the center of town."),
+        ]
+
+        async def fn() -> list[Message]:
+            return expected
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == expected
+
+    async def test_fn_returns_list_of_strings(self):
+        expected = [
+            "Hello, world!",
+            "I'm looking for a restaurant in the center of town.",
+        ]
+
+        async def fn() -> list[str]:
+            return expected
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [UserMessage(t) for t in expected]
+
+    async def test_fn_returns_resource_content(self):
+        """Test returning a message with resource content."""
+
+        async def fn() -> UserMessage:
+            return UserMessage(
+                content=EmbeddedResource(
+                    type="resource",
+                    resource=TextResourceContents(
+                        uri=FileUrl("file://file.txt"),
+                        text="File contents",
+                        mimeType="text/plain",
+                    ),
+                )
+            )
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(
+                content=EmbeddedResource(
+                    type="resource",
+                    resource=TextResourceContents(
+                        uri=FileUrl("file://file.txt"),
+                        text="File contents",
+                        mimeType="text/plain",
+                    ),
+                )
+            )
+        ]
+
+    async def test_fn_returns_mixed_content(self):
+        """Test returning messages with mixed content types."""
+
+        async def fn() -> list[Message]:
+            return [
+                UserMessage(content="Please analyze this file:"),
+                UserMessage(
+                    content=EmbeddedResource(
+                        type="resource",
+                        resource=TextResourceContents(
+                            uri=FileUrl("file://file.txt"),
+                            text="File contents",
+                            mimeType="text/plain",
+                        ),
+                    )
+                ),
+                AssistantMessage(content="I'll help analyze that file."),
+            ]
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(
+                content=TextContent(type="text", text="Please analyze this file:")
+            ),
+            UserMessage(
+                content=EmbeddedResource(
+                    type="resource",
+                    resource=TextResourceContents(
+                        uri=FileUrl("file://file.txt"),
+                        text="File contents",
+                        mimeType="text/plain",
+                    ),
+                )
+            ),
+            AssistantMessage(
+                content=TextContent(type="text", text="I'll help analyze that file.")
+            ),
+        ]
+
+    async def test_fn_returns_dict_with_resource(self):
+        """Test returning a dict with resource content."""
+
+        async def fn() -> dict:
+            return {
+                "role": "user",
+                "content": {
+                    "type": "resource",
+                    "resource": {
+                        "uri": FileUrl("file://file.txt"),
+                        "text": "File contents",
+                        "mimeType": "text/plain",
+                    },
+                },
+            }
+
+        prompt = Prompt.from_function(fn)
+        assert await prompt.render() == [
+            UserMessage(
+                content=EmbeddedResource(
+                    type="resource",
+                    resource=TextResourceContents(
+                        uri=FileUrl("file://file.txt"),
+                        text="File contents",
+                        mimeType="text/plain",
+                    ),
+                )
+            )
+        ]