[fix][client] Inject trace context into message properties for producer-consumer span correlation

[hrzzzz]

Motivation

The OpenTelemetry producer interceptor creates a send span but does not inject the trace context into message properties. On the consumer side, extractContext finds nothing in the properties and falls back to
Context.current(), so consumer spans always start a new trace instead of being children of the producer span. Producer and consumer spans are impossible to correlate.

Modifications

• Added TracingContext.injectContext(Message, Context, TextMapPropagator) overload that writes trace context directly to the message's underlying MessageMetadata protobuf, handling both MessageImpl and TopicMessageImpl wrappers.
• Called it from OpenTelemetryProducerInterceptor.beforeSend() after creating the producer span.

Verifying this change

• Make sure that the change passes the CI checks.

Added test: org.apache.pulsar.broker.service.OpenTelemetryTracingIntegrationTest#testContextPropagationViaMessageProperties

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• Dependencies (add or upgrade a dependency)
• The public API
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• The default values of configurations
• The threading model
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• Anything that affects deployment