fix(codecs): handle single-chunk GELF messages directly

[pront]

Summary

Motivation

A one-chunk GELF message is already complete, but the decoder sent it through the pending-message path. That created unnecessary state and made message-ID ownership harder to enforce.

Changes

• Decode one-chunk messages without creating pending state or a timeout task.
• Detach returned and retained payloads from the decoder input buffer.
• Reject a one-chunk message that reuses the ID of a pending multi-chunk message.

References

Related: #26137

Stack: #26299 -> #26300 -> #26301 -> #26302

Vector configuration

Not applicable; the decoder behavior is exercised directly.

How did you test this PR?

Focused tests exercise source-buffer ownership and pending message-ID reuse.

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.

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-03T19:00:16.433491Z	d750fd3	New commits

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