chore: improve rust-rdkafka maintenance path

[oab24413gmai]

Summary:

• Add focused unit tests to the existing #[cfg(test)] module in src/topic_partition_list.rs covering edge cases: inserting a duplicate (topic, partition) pair, iterating over an empty TopicPartitionList, setting offsets on partitions not yet added (verifying the documented error/None branch), and round-tripping through find_partition for a missing topic. Tests only — no behavior changes.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.