[connectors] input backpressure does not limit librdkafka prefetch, so a stalled parser causes OOM #7610

Cause

1. A RecvThread calls the parser, and the parser runs the preprocessor (input.rs:1475). If the preprocessor blocks, the thread stops reading its partition queues.
2. The backpressure thread pauses a connector only when is_full() is true (controller.rs:6638). is_full() compares buffered_records and buffered_bytes with the configured maximums (stats.rs:2587). The Kafka reader increments these counters only after it parses a record (input.rs:635, input.rs:749). The 1 MB per-partition limit also counts only parsed data (input.rs:1236).
3. InputReader::memory() gives the librdkafka queued bytes (kafka.rs:595). Its only use is the metric input_connector_extra_memory_bytes (controller.rs:2082).
4. RSS sets the memory-pressure level (runtime.rs:772). Memory pressure changes only the DBSP spill thresholds (for example runtime.rs:1430). It does not pause input connectors.
5. The reader splits each partition into its own queue (input.rs:550). With split queues, librdkafka applies queued.max.messages.kbytes (default 65536, 64 MiB) to each partition. For this pipeline the limit is 40 x 8 x 64 MiB = 20 GiB, which is more than the memory budget.