Add in-memory transport (#25)

## Goal

Support running an MCP server in the same process as the client, while preserving MCP abstractions.

## Details

1. **(core change)** Adds a new `memory` transport module that enables in-process client-server communication.
This includes:
   - `create_client_server_memory_streams()` to create bidirectional memory streams
   - `create_connected_server_and_client_session()` to establish an in-process client-server connection

3. (minor) Enhances error handling and timeout support:
   - Adds configurable read timeouts to sessions via `read_timeout_seconds` parameter
   - Improves exception handling in the server with a new `raise_exceptions` flag to control whether errors are returned to clients or raised directly
   - Ensures proper cleanup of request context tokens in error cases

4. (minor) Makes server improvements:
   - Adds built-in ping handler support

tests/conftest.py

@@ -0,0 +1,28 @@
+import pytest
+from pydantic import AnyUrl
+
+from mcp_python.server import Server
+from mcp_python.server.types import InitializationOptions
+from mcp_python.types import Resource, ServerCapabilities
+
+TEST_INITIALIZATION_OPTIONS = InitializationOptions(
+    server_name="my_mcp_server",
+    server_version="0.1.0",
+    capabilities=ServerCapabilities(),
+)
+
+@pytest.fixture
+def mcp_server() -> Server:
+    server = Server(name="test_server")
+
+    @server.list_resources()
+    async def handle_list_resources():
+        return [
+            Resource(
+                uri=AnyUrl("memory://test"),
+                name="Test Resource",
+                description="A test resource"
+            )
+        ]
+
+    return server