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

mcp_python/types.py

@@ -141,16 +141,19 @@ class ErrorData(BaseModel):
 
     code: int
     """The error type that occurred."""
+
     message: str
     """
     A short description of the error. The message SHOULD be limited to a concise single
     sentence.
     """
+
     data: Any | None = None
     """
     Additional information about the error. The value of this member is defined by the
     sender (e.g. detailed error information, nested errors etc.).
     """
+
     model_config = ConfigDict(extra="allow")