openai-v2: import OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT from genai-utils (#4455)

* openai-v2: import OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT from genai-utils

Remove local re-definition of OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT
and import it from opentelemetry.util.genai.environment_variables instead,
making genai-utils the single source of truth for this constant.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* openai-v2: add CHANGELOG entry for #4455

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Apply suggestion from @xrmx

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>