GenAI events - yamlify body definitions and cosmetic examples improvements

[lmolkova]

• Adds yaml definitions for GenAI events in preparation for Render events with bodies #1464
• Updates examples to be more concise and readable, fixes small inconsistencies, adds diagrams.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

let's clarify if these should be deserialized or provided as string.
The perf cost of deserialization compared to all other things necessary to create this event is small.

[trentm]

A concern: In general the "arguments" string returned by (at least OpenAI -- my experience is limited) may not be valid JSON. In the (likely rare) case that it isn't this means that "arguments" would then, presumably, fallback to be a string. That could be a rare surprise (i.e. possible bug) to whatever system is processing this telemetry.

I suppose that is fine if the type allows both string and object (or perhaps AnyValue would be used) -- then implementations need to cope.

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Another possible concern: https://platform.openai.com/docs/api-reference/chat/create#chat-create-response_format includes the (surprising to me) warning:

Important: when using JSON mode, you must also instruct the model to produce JSON yourself via a system or user message. Without this, the model may generate an unending stream of whitespace until the generation reaches the token limit, resulting in a long-running and seemingly "stuck" request.

Not having the raw "arguments" string in the telemetry could make it hard to debug why a parsed JSON response of, say {"foo": "bar"}, took 1024 (or however many) tokens.

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What are reasonable use cases for having the parsed arguments in telemetry?

[lmolkova]

discussed at GenAI call:

if max token is reached in the middle of the arguments, it's truncated, so let's keep it a string, but we might change it for some systems.

[xrmx]

Then we should also update the examples above

[lmolkova]

TODO: add generated md table into the description