receive_response() streams tool results as UserMessage, causing agent loops

## Problem

`receive_response()` streams MCP tool results as `UserMessage` containing `ToolResultBlock`, indistinguishable from real user messages without inspecting the content blocks. This makes it easy for the model to misinterpret tool results as new user input, causing infinite agent loops.

## Reproduction

When using `create_sdk_mcp_server` to register a custom MCP tool (e.g. `send_message`), the `receive_response()` stream looks like:

```
AssistantMessage → [ToolUseBlock(name='mcp__clawless__send_message', ...)]
UserMessage → [ToolResultBlock(tool_use_id='...', content='Message sent')]
AssistantMessage → [ToolUseBlock(name='mcp__clawless__send_message', ...)] # agent loops
UserMessage → [ToolResultBlock(...)]
... repeats indefinitely
```

The agent sees the `UserMessage` (tool result) and interprets it as requiring a response, calling the tool again, creating an infinite loop.

## Expected behavior

One of:

1. **Distinct message type**: Stream tool results as a `ToolResultMessage` (or similar) rather than `UserMessage`, so consumers can differentiate without inspecting content blocks.
2. **Filtered stream**: Provide an option to only yield "meaningful" messages (assistant text, result) and handle tool round-trips internally.
3. **Documentation**: At minimum, document that `UserMessage` in the stream may contain `ToolResultBlock` and is not always a real user message.

## Workaround

We worked around this with:
- System prompt instructions explicitly telling the model that tool results are not user messages
- Content validation in the tool handler (rejecting trivially short messages)
- Per-turn rate limiting on tool calls

These are defense-in-depth but the root cause is the ambiguous message typing.

## Environment

- `claude-agent-sdk` (Python)
- MCP tools via `create_sdk_mcp_server`