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 ✨

Integrations

• feat(integrations): OpenAI/OpenAI Agents detect and report the time to first token metric (TTFT) as gen_ai.response.time_to_first_token by constantinius in #5348
• feat(integrations): openai-agents streaming support by constantinius in #5291

Openai

• feat(openai): Set system instruction attribute for Responses API by alexander-alderman-webb in #5376
• feat(openai): Set system instruction attribute for Completions API by alexander-alderman-webb in #5358

Openai Agents

• feat(openai-agents): Set system instruction attribute on gen_ai.chat spans by alexander-alderman-webb in #5370
• feat(openai-agents): Set system instruction attribute by alexander-alderman-webb in #5355

Other

• feat(anthropic): Set system instruction attribute by alexander-alderman-webb in #5353
• feat(asyncio): Allow to turn task spans off by sentrivana in #5367
• feat(gen_ai): add function set_conversation_id and managing functions on the Scope and apply it on the Span on .finish() by constantinius in #5362
• 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(pydantic-ai): Set system instruction attribute by alexander-alderman-webb in #5356
• feat(transport): Report 413 responses for oversized envelopes by alexander-alderman-webb in #5380

Bug Fixes 🐛

• 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
• ref: Replace set_data_normalized() with Span.set_data() for system instructions by alexander-alderman-webb in #5374
• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

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