[processor/genainormalizer] Fix OpenInference flattened message normalization

[Jwrede]

Summary

Fixes #48421

OpenInference represents messages as flattened indexed span attributes (llm.input_messages.0.message.role, llm.input_messages.0.message.content, etc.) but the processor only performed exact key matching against llm.input_messages, which never appears as a literal attribute key. This was a silent no-op for all message normalization.

Changes:

• Add message reconstruction that collects flattened llm.{input,output}_messages.N.message.* attributes and rebuilds them into GenAI semconv JSON (gen_ai.input.messages / gen_ai.output.messages)
• Support text content, tool call requests, and tool call responses
• Remove broken exact-match entries from the lookup table
• Add attributeAggregator interface to the processor for multi-attribute-to-one transformations
• Add unit tests and end-to-end test matching the real-world format from the issue

Test plan

• Unit tests for message reconstruction (simple messages, tool calls, overwrite/remove behavior)
• End-to-end test through createTracesProcessor with the exact attribute format from the issue
• All existing tests pass unchanged
• go vet ./... clean

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[TylerHelmuth]

@Jwrede the genainormalizer is still being donated to Contrib (see #46069). We are opening issues as we add the component, but we are not ready for those issues to be worked. The component will likely continue to undergo serious change as it is being donated and we aren't ready to accept other PRs for it yet. I'm going to close this for now, but look for the issue to be workable once the on-hold label is removed.

[Jwrede]

Thanks for the context @TylerHelmuth -- makes sense. I will hold off until the donation is complete and the component stabilizes.