fix: raise 400 error for invalid render format

[vincentsarago]

Technical Context

• Related Issues: closes InvalidFormat returns 500 instead of 400 #1493
• Breaking Changes: No

Description

Provide a brief summary of the changes, the rationale behind them, and any specific areas you'd like the reviewers to focus on.

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Checklist

• Linting: Code is formatted and linted (uv run pre-commit run --all-files)
• Tests: Tests pass. I have included new tests for these changes where applicable.
• Edge Cases: I have manually verified "unhappy paths" and edge cases beyond the basic success criteria (e.g., database connection timeouts, malformed input, strict mapping rejections).
• Documentation: I have updated README.md to reflect any new environment variables, configuration changes, or breaking updates.
• Accountability: I can explain the implementation logic for every line of code submitted.

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AI tool usage

• AI (Copilot or something similar) supported my development of this PR. Use of AI tools HAVE TO be indicated.

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Policy: We require a "human-in-the-loop." You are the author and are fully accountable for all submitted code. Please ensure all tool-generated content is thoroughly reviewed before submission to ensure it is not an "extractive contribution" that squanders maintainer time.