fix(openai): Simplify Completions input handling

[alexander-alderman-webb]

Description

Issues

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

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Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Bug Fixes 🐛

• (openai) Simplify Completions input handling by alexander-alderman-webb in #5690

Internal Changes 🔧

• (openai) Move input handling code into API-specific functions by alexander-alderman-webb in #5687

Other

• Update CHANGELOG.md by sentrivana in #5685

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Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 6.86s

All tests are passing successfully.

❌ Patch coverage is 0.00%. Project has 14347 uncovered lines.

Files with missing lines (1)

File	Patch %	Lines
openai.py	4.45%	⚠️ 623 Missing

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