🧹 Refactor _get_model_or_400 into common helper function

[chottokun]

🎯 What:
Extracted duplicated model lookup and 400 error handling logic into a unified get_model_or_400 helper function in src/app/models.py, and updated src/app/services/embedding.py, src/app/services/rerank.py, and src/app/main.py to use it.

💡 Why:
Previously, _get_model_or_400 was duplicated across embedding.py, rerank.py, and main.py. Consolidating this logic into a single helper reduces code duplication, improves maintainability, and ensures consistent error handling and formatting across services.

✅ Verification:
Ran full linting (uv run ruff check .), formatting (uv run ruff format .), and test suite (uv run pytest). All 149 tests passed without regression.

✨ Result:
Cleaned up duplicate code across services while maintaining 100% backward compatibility and test coverage.

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PR created automatically by Jules for task 23163263657246516 started by @chottokun

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[chottokun]

本PRのリファクタリング内容（_get_model_or_400 の共通化）は、PR #88 にてプレフィックス重複排除やDocstring拡充とともに包括的に統合・ブラッシュアップを進めています。PR #88 のマージ完了後に本PRをクローズ予定です。

[google-labs-jules]

本PRのリファクタリング内容（_get_model_or_400 の共通化）は、PR #88 にてプレフィックス重複排除やDocstring拡充とともに包括的に統合・ブラッシュアップを進めています。PR #88 のマージ完了後に本PRをクローズ予定です。

承知いたしました。ご共有ありがとうございます。PR #88 での統合・クローズ処理について理解いたしました。