Introduce per-message structured GenAI events instead of prompt/completion span events #5594

Job	Run time
markdown-link-check	4m 27s
areas-dropdown-check	3s
markdownlint	22s
markdown-toc-check	36s
misspell	19s
policies-check	51s
schemas-check	19s
semantic-conventions	10s
semantic-conventions-compatibility	14s
semantic-conventions-registry	28s
yamllint	13s
	8m 2s

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