feat(gateways): map Kafka records to Iggy messages and back

[krishvishal]

Part of #3560. This is the record codec that #3535 (Produce) and #3536 (Fetch) both sit on, and it writes the mapping document that was the last open scope item of #3533.

to_iggy takes one Kafka record and returns one Iggy message. The value becomes the payload. The key and each header become kafka. user headers. A null or empty value is stored as a single placeholder byte plus a marker that says which of the two it was, so a tombstone stays on the fast path. Kafka counts timestamps in milliseconds and Iggy counts them in microseconds, so the value is multiplied by 1000 on the way in and divided on the way out.

Some records do not fit. Iggy rejects an empty header value, caps a name and a value at 255 bytes, and caps all headers of one message at 100 KB. A record that trips any of those is written into an envelope instead, at the byte layout the document fixes. from_iggy reads both forms, and a caller cannot tell them apart.

A message that no Kafka producer wrote is handled too. If it carries no kafka. headers, an Iggy client wrote it, so it becomes a record with a null key and its own headers.

The batch layer sits on top. decode_batches drains a whole partition blob, because one blob holds several batches back to back. encode_batch writes one uncompressed batch for Fetch. Reading a compressed batch needs the four codecs, so gzip, lz4, snappy and zstd are turned back on for kafka-protocol.

Compressed input needs a limit. max_frame_size only bounds the compressed bytes, and zstd reaches 1000 to 1 on repetitive input without being asked. One frame also holds up to 4096 partition entries, so a limit applied to each batch on its own still admits 4096 times that much. The budget therefore covers the whole request.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (7bae8ec) to head (e110b04).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4229      +/-   ##
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- Coverage     87.64%   86.43%   -1.21%     
  Complexity     1575     1575              
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  Files          1284     1283       -1     
  Lines        225457   217064    -8393     
  Branches     188820   180429    -8391     
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- Hits         197593   187611    -9982     
- Misses        23135    24512    +1377     
- Partials       4729     4941     +212     

Components	Coverage Δ
Rust Core	87.31% <ø> (-1.44%)	⬇️
Java SDK	68.68% <ø> (ø)
C# SDK	77.41% <ø> (+0.06%)	⬆️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.43% <ø> (+0.06%)	⬆️
Go SDK	70.11% <ø> (+0.05%)	⬆️
see 186 files with indirect coverage changes

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[ryerraguntla]

Looks good. Fix the header_count cap and few other minors.

[ryerraguntla]

Mention the issue it is trying to fix and close. Also PR needs to be in the expected format with the required details.

All review comments addressed.