dekaf: Fix ReadRequest to return journal write head

[jshearer]

Description:

Fixes a small but important mistake in #2358, namely that this actually does not work as a way to get a metadata response from the broker in order to fetch the write head of a journal:

broker::ReadRequest {
    journal: partition.spec.name.clone(),
    offset: -1,
    ..Default::default()
};

Instead, the proper incantation is

broker::ReadRequest {
    journal: partition.spec.name.clone(),
    offset: 0,
    metadata_only: true,
    ..Default::default()
};

Tested locally and on dekaf-dev, confirmed this now doesn't error when fetching offsets with a timestamp of -1.

$ kcat -o beginning  -b dekaf-dev.estuary-data.com:9092 \                                                                                                                                                                                                                                                                   
-X security.protocol=SASL_SSL \
-X sasl.mechanism=PLAIN \
-Q -t "events:0:-1"
events [0] offset 414002872

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This change is 

[jgraettinger]

LGTM