[Rust] Add mux acknowledgment callback bridge

[danilonajkov-db]

What changes are proposed in this pull request?

This PR implements the acknowledgment callback bridge required by multiplexed streams.

• Generalize AckCallback over its identifier type while preserving OffsetId as the default, so existing individual-stream callback implementations remain unchanged.
• Add a crate-private multiplexed callback adapter that captures one sub-stream index.
• Convert each acknowledged stream-local OffsetId into the MessageId exposed by MultiplexedStream.
• Forward both successful acknowledgments and errors through AckCallback<MessageId>.
• Validate that callback adapters are only created for supported sub-stream indices.

The multiplexed stream builder is being added separately in #433. Its only callback-specific responsibility will be to install one adapter per sub-stream:

let callback = multiplexed_ack_callback(stream_index, callback.clone());
grpc_config.ack_callback = Some(callback);

This gives multiplexed streams the same user-facing callback API as individual streams, except the identifier is MessageId instead of OffsetId:

impl AckCallback<MessageId> for MyCallback {
    fn on_ack(&self, message_id: MessageId) {
        println!("Acknowledged {message_id}");
    }

    fn on_error(&self, message_id: MessageId, error_message: &str) {
        eprintln!("Error for {message_id}: {error_message}");
    }
}

Each adapter already knows which sub-stream it belongs to, so callback delivery requires no lookup, routing table, or locking. It only packs the captured stream index and acknowledged offset into a MessageId before invoking the user callback.

How is this tested?

• Verifies callbacks from different sub-streams produce different MessageId values for the same stream-local offset.
• Verifies errors preserve both the correct MessageId and error message.
• Verifies invalid sub-stream indices are rejected.
• cargo test -p databricks-zerobus-ingest-sdk
• cargo clippy -p databricks-zerobus-ingest-sdk --all-targets -- -D warnings
• cargo fmt --all --check

[danilonajkov-db]

@elenagaljak-db's comments from previous PR

[danilonajkov-db]

@elenagaljak-db this should be much cleaner now, we have the adapter/bridge to translate the message_id in mux streams to the offset id in individual streams and made the ack callback IDs generic (so we can reuse the same one for both mux and normal streams)

[teodordelibasic-db]

LGTM

[teodordelibasic-db]

nit: I don't think this is needed.

[danilonajkov-db]

ill remove it anyway when I integrate this in the stream builder