Fix MCP async tool_runner docs

[fallintoplace]

Problem

Some MCP async examples show await on client.beta.messages.tool_runner(...), but tool_runner() returns an async iterator and is not awaitable.

Users who copy that pattern hit a runtime TypeError before entering the runner loop.

Fix

• Remove the incorrect await in the MCP helpers guide example (helpers.md).
• Remove the incorrect await from the async_mcp_tool() docstring example (src/anthropic/lib/tools/mcp.py).

Both examples now directly assign runner = client.beta.messages.tool_runner(...) and then iterate with async for.

Testing

Not run (docs-only change).