Streamlines the vendored kafkajs instrumentation onto Sentry's span APIs.

Span creation moves from the OTel tracer to `startInactiveSpan`/`withActiveSpan`,
and cross-broker trace propagation now uses `getTraceData` (producer inject) and
`continueTrace` (consumer `eachMessage`) instead of the OTel propagator, so the
vendored `bufferTextMapGetter` is gone. The batch (`eachBatch`) path keeps the
upstream shape: the `poll` span is a fresh root and each processed message links
back to its producer span.

The four OTel metrics (no MeterProvider is wired up) and the `network.request`
event listeners that only fed them are dropped, along with `recordException` and
the producer/consumer hooks (origin is folded into span creation instead). Helpers
are extracted into a `utils.ts` and the now-trivial `types.ts`/`internal-types.ts`
are removed.

Adds a real error-path scenario to the integration suite asserting the producer
span is marked errored when a send fails.