fix: update BetaAsyncAbstractMemoryTool docstring to use async example

[yuribodo]

Summary

The BetaAsyncAbstractMemoryTool docstring example was copy-pasted from the synchronous BetaAbstractMemoryTool without being updated for async usage. Following the example verbatim raises TypeError at instantiation time.

Changes

• Use BetaAsyncAbstractMemoryTool as the base class (was BetaAbstractMemoryTool)
• Add async def to method definitions (was def)
• Replace Anthropic() with AsyncAnthropic()
• Wrap usage in async def main() / asyncio.run(main())

Before (broken)

class MyMemoryTool(BetaAbstractMemoryTool):  # wrong base class
    def view(self, command: ...):             # missing async
        ...
client = Anthropic()                          # sync client

After (fixed)

class MyMemoryTool(BetaAsyncAbstractMemoryTool):
    async def view(self, command: ...):
        ...
client = AsyncAnthropic()
await client.beta.messages.run_tools(...)

Fixes #1290