Fix producer max pending messages validation inconsistency

[AmiradelBeyg]

Summary

ProducerBuilder.Create() rejects MaxPendingMessages = 0, but
PulsarClient.CreateProducer(ProducerOptions<TMessage>) did not.

This made producer creation behavior inconsistent depending on which API path was used. It also allowed an invalid
configuration to reach the internal send queue, which could hide problems until high-volume or production usage.

Changes

• add missing MaxPendingMessages > 0 validation in PulsarClient.CreateProducer
• keep the existing builder behavior unchanged
• add unit coverage for both producer creation paths

Why

Both producer creation APIs should enforce the same contract. Rejecting invalid configuration early is clearer and
prevents subtle runtime issues caused by a zero-capacity pending message limit.

Testing

• added unit test for PulsarClient.CreateProducer(ProducerOptions<TMessage>)
• added unit test for ProducerBuilder.Create()
• ran:

dotnet test tests/DotPulsar.Tests/DotPulsar.Tests.csproj --filter "PulsarClientProducerValidationTests|
ProducerBuilderTests"

## Notes

I did not add an integration test for MaxPendingMessages because there was no existing integration coverage for this
option, and the bug is a validation/contract issue at the API boundary.

[blankensteiner]

Thanks for the PR. Two small corrections and I can merge it :-)

[blankensteiner]

This class should just be called "PulsarClientTests", otherwise everything is fine :-)

[AmiradelBeyg]

I can rename that class, but there is already an existing integration test class named PulsarClientTests in this test project. Do you want me to rename the new unit test class and also rename the existing integration class, or would you prefer I keep the current name to avoid duplicate types?

[blankensteiner]

The convention is that if we have a class called DotPulsar.SomeNamespace.ClassName, then we can find all tests for that class in DotPulsar.Tests.SomeNamespace.ClassNameTests, so if there is already a class with that name, then just add the new tests to that class :-)

[AmiradelBeyg]

Done!

[blankensteiner]

This class should be moved to the "Internal" namespace :-)

[AmiradelBeyg]

Done!

[blankensteiner]

Thanks! :-)