fix(streaming): guard against out-of-bounds content index; fix BetaAsyncAbstractMemoryTool docstring

[rmotgi1227]

Two fixes in this PR.

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Fix 1 — IndexError when content_block_delta/stop arrives before content_block_start (fixes #1192)

Files: src/anthropic/lib/streaming/_messages.py, src/anthropic/lib/streaming/_beta_messages.py

accumulate_event indexes directly into current_snapshot.content[event.index] for both content_block_delta and content_block_stop events. If the server sends a delta or stop for an index that hasn't been populated by a preceding content_block_start (server error, reconnect, or race condition), an unhandled IndexError propagates to the caller.

Add a bounds guard and return the snapshot unchanged in that case:

# BEFORE
elif event.type == "content_block_delta":
    content = current_snapshot.content[event.index]

# AFTER
elif event.type == "content_block_delta":
    if event.index >= len(current_snapshot.content):
        return current_snapshot
    content = current_snapshot.content[event.index]

Applied to both content_block_delta and content_block_stop in both _messages.py and _beta_messages.py (4 guards total).

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Fix 2 — BetaAsyncAbstractMemoryTool docstring is a copy-paste of the sync class (fixes #1290)

File: src/anthropic/lib/tools/_beta_builtin_memory_tool.py

The docstring for BetaAsyncAbstractMemoryTool was a verbatim copy of BetaAbstractMemoryTool's docstring, instructing users to:

• Subclass BetaAbstractMemoryTool (the sync class) ❌
• Use def view / def create (sync methods) ❌
• Use Anthropic() (sync client) ❌

Following the example verbatim raises TypeError at runtime. Updated to show the correct async class name, async def methods, AsyncAnthropic client, and asyncio.run() wrapper.