bug(Rust SDK): on merge of ShardMessages the buffer budget is freed before messages are send

[haubur]

Bug description

Situation
The ShardMessageWithPermit has inner Vec for which size the _permit_bytes are taken.
Merging the batches in appends IggyMessages to the Vec without touching the _permit_bytes.
The loop drops on iteration and thereby frees the permit.

iggy/core/sdk/src/clients/producer_sharding.rs

Lines 227 to 235 in 3db955e

	for msg in buffer.drain(..) {
	if let Some(last) = merged_batches.last_mut()
	&& Self::same_destination(&last.inner, &msg.inner)
	{
	last.inner.messages.extend(msg.inner.messages);
	continue;
	}
	merged_batches.push(msg);
	}

Consequence
The actual bytes in flight can exceed max_buffer_size and the BackpressureMode does not kick.

Affected area / component

Rust SDK

Deployment

None

Versions

No response

Hardware / environment

No response

Sample code

No response

Logs

No response

Iggy server config

No response

Reproduction

No response

Contribution

• I'm willing to submit a pull request to fix this bug

Good first issue

• I think this could be a good first issue for a new contributor