fix(ai): Keep single content input message

[alexander-alderman-webb]

Description

Store only one message from the conversation history on the gen_ai.request.messages attribute.

Keep prior logic that progressively trims away messages only for embeddings. Removes tests that check the existence of conversation histories or prompt messages that are no longer set on the attribute.

Issues

Closes #5335

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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New Features ✨

• feat(anthropic): Set system instruction attribute by alexander-alderman-webb in #5353
• feat(google-genai): Set system instruction attribute by alexander-alderman-webb in #5354
• feat(langchain): Set system instruction attribute by alexander-alderman-webb in #5357
• feat(openai-agents): Set system instruction attribute by alexander-alderman-webb in #5355
• feat(pydantic-ai): Set system instruction attribute by alexander-alderman-webb in #5356

Bug Fixes 🐛

Ai

• fix(ai): Keep single content input message by alexander-alderman-webb in #5345

• fix(ai): Keep single content input message by alexander-alderman-webb in #5345

Internal Changes 🔧

• ci(release): Fix changelog-preview permissions by BYK in #5368
• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

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