Fix field count encoding in DescribedList and add tests for full field count compliance

[kamil-mrzyglod]

DescribedList.EncodeValue() previously stopped encoding at the last non-null field, which is permitted by AMQP 1.0 spec section 1.4 but breaks non-.NET clients (Python _pyamqp, uamqp) that access performative fields by fixed numeric index — causing IndexError when the encoded list is shorter than expected.

This change replaces the last-set-bit calculation with this.fieldCount, so all defined fields are always written (unset ones as null bytes). This matches the behaviour of the real Azure Service Bus / Event Hubs broker.

References: TheCloudTheory/Topaz#210.

[xinchen10]

Since this is permitted by AMQP 1.0 spec, should we fix the other clients instead?

[kamil-mrzyglod]

@xinchen10 I am not in a position to influence the direction, but IMHO fixing other clients isn't the solution. Yes, AMQP spec allows the existing behaviour but other SDKs fail to comply with AMQPNet for a reason. To me the proposed fix is a much better solution - it fixes the issue centrally, is simple and the change is strictly additive.

[xinchen10]

Thanks for bringing up the issue. I am reaching out to the SDK owners to get it fixed there. Azure SDKs should be protocol compliant. AMQP is a standard and has more clients than just Azure SDKs. The Apache qpid-proton-j does same compact encoding of composite list types.

[kamil-mrzyglod]

Thanks @xinchen10 :)