fix(streaming): replace IndexError with descriptive RuntimeError for out-of-bounds content_block_delta

[tao-to-python]

Description

Improves error diagnostics for intermittent IndexError during streaming, as reported in #1192.

Problem

In accumulate_event(), current_snapshot.content[event.index] can raise an opaque IndexError when the index exceeds the content list length (e.g. due to a missing content_block_start event or a race condition).

The raw IndexError provides no context about what happened.

Solution

Add bounds checking before the index access with a descriptive error:

if event.index >= len(current_snapshot.content):
    raise RuntimeError(
        f"Received content_block_delta for index {event.index}, "
        f"but snapshot only has {len(current_snapshot.content)} content block(s). "
        f"This may indicate a missing content_block_start event."
    )

Applied to both _messages.py and _beta_messages.py.

Fixes #1192