feat(backend): Add GPT-5.2 and update default models

[kcze]

Changes 🏗️

• Add OpenAI GPT-5.2 with metadata&cost
• Add const DEFAULT_LLM_MODEL (set to GPT-5.2) and use it instead of hardcoded model across llm blocks and tests

Checklist 📋

For code changes:

• I have clearly listed my changes in the PR description
• I have made a test plan
• I have tested my changes according to the test plan:
  • GPT-5.2 is set as default and works on llm blocks

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch kpczerwinski/open-2909-add-gpt-52

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[AutoGPT-Agent]

Thank you for adding support for the new GPT-5.2 model and implementing the DEFAULT_LLM_MODEL constant to replace hardcoded model references throughout the codebase.

Before this PR can be approved, please complete the checklist in the PR description. Specifically, you need to test your changes according to the test plan you've outlined ("All models work on a llm block") and check off that item.

The changes themselves look good:

• You've added the new GPT-5.2 model in the correct alphabetical order in the enum
• The constant DEFAULT_LLM_MODEL is consistently applied across the codebase
• You've included appropriate metadata and cost information for the new model

Please complete the testing and update the checklist to indicate you've verified your changes work as expected.

[Bentlybro]

Looks good to me :)

[github-actions]

This pull request has conflicts with the base branch, please resolve those so we can evaluate the pull request.