Improve Surrogate-Conti Limitations

[Scienfitz]

Implements #658

[Copilot]

Pull Request Overview

This PR improves the handling of surrogate model limitations when working with continuous parameters by replacing a blanket restriction with proper error handling and user feedback. The changes address issue #658 by allowing surrogate models to attempt gradient computation and gracefully failing with informative error messages when they cannot support continuous spaces.

Key changes:

• Replaces upfront surrogate compatibility checks with runtime error handling
• Adds comprehensive test coverage for surrogate-continuous parameter incompatibilities
• Updates documentation to reflect the improved error handling approach

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
tests/test_surrogate.py	Adds parametrized test to verify proper error handling for incompatible surrogate-continuous parameter combinations
docs/userguide/constraints.md	Removes outdated warning about silent constraint ignoring
baybe/surrogates/base.py	Removes blanket restriction preventing non-GP surrogates from being used with continuous spaces
baybe/recommenders/pure/bayesian/base.py	Adds try-catch logic to detect gradient computation failures and raise informative error messages

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[AVHopp]

LGTM