refactor(codecs): assemble final GELF chunk directly

[pront]

Summary

Motivation

Final GELF chunk assembly was coupled to mutable pending-message state, which made the later resource-accounting changes harder to reason about.

Changes

• Consume the completed message state before assembling the output.
• Cancel its timeout before releasing the message-ID lock so an old callback cannot remove a newly reused ID.
• Verify timeout callback identity before removal in case cancellation races a callback already waiting on the lock.
• Insert the final chunk directly in sequence instead of storing and copying it again.
• Keep the existing per-message length behavior unchanged.

References

Related: #26137

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

Vector configuration

Not applicable; this refactors the decoder internals.

How did you test this PR?

Focused chunked GELF tests cover ordered, unordered, and final-chunk assembly, including timeout cancellation.

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 2026-09-03T19:25:19.565897Z	415596a	Manual request

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[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 1961f0ae35

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[pront]

@codex review again

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: 415596a852

ℹ️ About Codex in GitHub

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• Comment "@codex review".

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