fix(codecs): three pre-existing chunked_gelf bugs

[pront]

Summary

Three independent bugs in the chunked_gelf framing decoder, split out of #26137 so that PR is left with only the new memory bound it is really about. All of these affect master today.

A one-byte message panics the source. decode_message formats the first two bytes of a frame in its trace log, but the guard above only rejects an empty one, so a single-byte datagram panics with range end index 2 out of range. It needs trace logging enabled for the target, since tracing only evaluates the field expression when the level is on, but it is reachable from the same unauthenticated path as everything else the decoder handles. Introduced in #21816.

pending_messages_limit is applied to every chunk. It is checked before the table lookup, so once the table is full the decoder rejects every chunk, including chunks belonging to messages already pending. Those can then never complete and expire instead, so a burst of new message IDs stalls legitimate in-flight traffic rather than merely capping how much is buffered. Only a new message grows the table, so the check belongs on insert.

Dropping a message for exceeding max_length leaks its timeout task. The entry is removed without aborting the timer spawned for it, so the task outlives the entry it was meant to reclaim and the live-task count is not bounded by pending_messages_limit the way the entry count is.

Also here

Memory hygiene in the same file, no behaviour change:

• BytesDecoder hands over a slice of FramedRead's buffer without copying, so storing that slice held the whole buffer for as long as the chunk was pending. The buffer is at least 8 KiB and an MTU-sized chunk is around 1388 bytes, so a pending message could hold several times what its payload needs. add_chunk now copies.
• Reassembly reserved nothing and grew into a BytesMut, paying reallocation slack on top of the copy it already needs. It now reserves the length up front and releases each chunk as it copies.
• HashMap stores values inline and MessageState is around 4 KiB because of its fixed chunk array, so every bucket carried one whether occupied or not and a rehash transiently allocated two copies of the table. Boxed.

References

Split out of #26137.

Vector configuration

sources:
  graylog:
    type: socket
    mode: udp
    address: 0.0.0.0:12201
    framing:
      method: chunked_gelf
    decoding:
      codec: gelf

sinks:
  out:
    type: blackhole
    inputs:
      - graylog

How did you test this PR?

Five unit tests in lib/codecs/src/decoding/framing/chunked_gelf.rs, one per fix. Each was checked against a deliberately broken build to confirm it fails, rather than assumed to work: the panic test reproduces range end index 2 out of range before the fix, and reverting the copy makes the stored chunk alias the input buffer.

make fmt, cargo clippy -p codecs --lib --tests, make check-generated-docs and the full codecs lib suite are clean.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.