fix(llmobs): swallow LLMObsAnnotateSpanError on auto-annotation in @llm decorator

[ZStriker19]

Summary

• Fixes a regression introduced in fix(llmobs): auto-annotate output for @llm decorator #16892 where the @llm decorator raised LLMObsAnnotateSpanError: Failed to parse output messages when a decorated function returned a value that couldn't be parsed as LLM messages (e.g. a plain string, integer, or non-messages dict).
• The decorator now catches LLMObsAnnotateSpanError from auto-annotation, logs a warning, and continues — the user's function still succeeds and the span is still created.
• Also adds the missing operation_kind != "embedding" guard from the 4.6 backport branch to main.

Test plan

• Two regression tests added (sync + async) verifying the warning is logged and no exception is raised
• Full lint checks pass

[cit-pr-commenter-54b7da]

Codeowners resolved as

ddtrace/llmobs/decorators.py                                            @DataDog/ml-observability
releasenotes/notes/llm-decorator-auto-annotation-error-a9ff1d25e3706cd3.yaml  @DataDog/apm-python
tests/llmobs/test_llmobs_decorators.py                                  @DataDog/ml-observability

[Yun-Kim]

let's shorten the release note but otherwise lgtm

[github-actions]

This change is marked for backport to 4.6 and it does not conflict with that branch.
The command used to test backporting was

git checkout 4.6 && git cherry-pick -x --mainline 1 b2748b379d73cdd3210112ef11c38f2d17b3e460

[ZStriker19]

/merge

[gh-worker-devflow-routing-ef8351]

View all feedbacks in Devflow UI.

2026-03-24 19:44:31 UTC ℹ️ Start processing command /merge

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2026-03-24 21:03:16 UTC ℹ️ MergeQueue: merge request added to the queue

The expected merge time in main is approximately 2h (p90).

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2026-03-24 21:47:58 UTC ℹ️ MergeQueue: This merge request was merged