fix(bedrock): Correctly set index on anthropic content attribute

[RileyEv]

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[RileyEv]

This won't work with tool calling. However, there is some better logic for reading the message and converting to span attributes in the opentelemetry-instrumentation-anthropic library, which does support tool calling. Is it okay to add a dependency, or should I just pull this logic out and add to this library too?

[nirga]

Nice catch @RileyEv! Mind signing the CLA?

We can pull that dependency, or better yet - extract it to the ai semconv common library