fix: Deny reader creation for partitioned topics

[PookieBuns]

Right now during reader creation it validates and deny's multi topics via

pulsar-rs/src/consumer/builder.rs

Line 334 in 86fda88

if self.topics.unwrap().len() > 1 {

but this catch fails to account for partitioned topics which returns multiple joined_topics. This creates unpredictable behavior at

pulsar-rs/src/consumer/builder.rs

Line 340 in 86fda88

let (topic, addr) = joined_topics.pop().unwrap();

since it will choose the first partition randomly depending on how its returned by the broker during lookup.

This change changes the validation of topic len on joined_topics.len() instead of self.topics to prevent unpredictable behavior.

If we want to support partitioned topics in pulsar-rs I can draft up a separate PR that internally changes the Reader to hold a InnerConsumer to account for partitioned topics