[Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example)

[Link to Advanced Usage Docs Example
](https://platform.claude.com/docs/en/agents-and-tools/tool-use/tool-runner#:~:text=(block.text)-,Advanced%20usage,-Within%20the%20loop)
# Root cause

Calling `runner.append_messages(user_message)` inside the loop sets
`_messages_modified = True`, which causes `__run__` to skip the auto-append
of the assistant message + tool result (line 281 of `_beta_runner.py`).
The next iteration sends a request with no tool result in history, so Claude
sees the original question unanswered and makes the tool call again.

## Minimal repro

Compacted and consolidated from the website

```
from anthropic import beta_tool, Anthropic

client = Anthropic()

@beta_tool
def get_weather(location: str, unit: str = 'fahrenheit') -> str:
return json.dumps({
'temperature': '20°c', 'condition': 'sunny'
})

runner = client.beta.messages.tool_runner(
model="claude-haiku-4-5",
max_tokens=1024,
tools=[get_weather],
messages=[{
"role": "user",
"content": "what's the weather in San Francisco?"
}]
)

for message in runner:
tool_response = runner.generate_tool_call_response()
if tool_response:
print(f"Tool result: {tool_response}")

# increase tokens for next request
runner.set_messages_params(
lambda params: {
**params,
"max_tokens": 2048,
}
)

# add additional messages
runner.append_messages(
{"role": "user", "content": "be concise in your response"}
)
```

## Expected behavior
The "be concise" user message is appended, tool results are still
auto-appended, loop terminates.

## Actual behavior
Infinite loop — tool result never reaches history.

## Note
The "Modifying tool results" example on the same page correctly calls
`runner.append_messages(message, tool_response)` (both the assistant
message and the tool result), which is the pattern that works correctly.

The "Advanced usage" example is inconsistent with this.