chore(llmobs): make openai agents trace span input the last message (#13198)

Makes the OAI agent workflow span (representing the overall trace) have
its input value be the last (not the first) message in the first LLM
call made. This isn't a breaking change, and is already safely handled
(in case the input messages list is empty).

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

Author: Yun-Kim

ddtrace/llmobs/_integrations/utils.py

@@ -627,7 +627,7 @@ def llmobs_trace_input(self) -> Optional[str]:
         try:
             messages, _ = self.llmobs_input_messages()
             if messages and len(messages) > 0:
-                return messages[0].get("content")
+                return messages[-1].get("content")
         except (AttributeError, IndexError):
             from ddtrace.internal.logger import get_logger