Update OpenAI instrumentation to latest sem conv + use genai utils as much as possible

What problem do you want to solve?

OpenAI instrumentation carries mixed semantic-convention behavior, with legacy and latest GenAI semconv paths coexisting. The goal is to fully migrate to latest GenAI semconv behavior and remove legacy semconv logic.

Describe the solution you'd like

PR Plan

PR1: Remove legacy chat completion paths only, leaving embeddings and responses behavior unchanged. - https://github.com/open-telemetry/opentelemetry-python-genai/pull/108
PR2: Migrate embeddings instrumentation to util-genai lifecycle and drop embeddings-specific legacy routing/plumbing.
PR3: Clean up responses instrumentation wrappers/flow for latest-semconv consistency while keeping non-responses paths untouched. - https://github.com/open-telemetry/opentelemetry-python-genai/pull/92
PR4: Rebaseline tests and docs/examples for the final support model, then run conformance and final validation.

Describe alternatives you've considered

No response

Additional Context

No response

Would you like to implement it?

Yes

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