Introduce resource preparation

[jeipollack]

Summary

Introduce resource preparation as part of resource resolution, allowing missing required resources to be prepared through registered resource preparers.

Closes #249

What’s changed

• Add RESOURCE_PREPARERS registry for resource preparation callables
• Add prepare_resources helper to invoke resource preparers with resource configuration and dataset
• Extend resolve() to prepare missing resources and combine them with caller-provided resources
• Add/update unit tests for resource preparation and resource resolution
• Extend RESOURCES_DEFAULT in conftest.py to support resource preparation tests

How to test / verify

• Run tests in resources_test.py
• CI passes.

Scope

Indicate the type of PR:

• Feature
• Bug fix
• Hotfix
• Documentation / process change
• Internal / refactor
• Release

Optionally, note if this PR is part of a larger milestone or set of related PRs.

Changelog

Did this PR introduce user-visible changes?
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• Changelog fragment added (if applicable)

Reviewer Checklist

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• The PR targets the correct base branch (develop, or main for release PRs)
• The PR is assigned to the developer
• Appropriate labels are applied
• The PR is included in relevant projects and/or milestones
• Description clearly explains what has changed
• Issue references included, if applicable
• Code and documentation adhere to current standards (ruff)
• Documentation updates included, if relevant
• CI tests are passing
• All reviewer comments have been addressed

Next Steps / Notes (if applicable)

The resource-preparation mechanism currently provides the orchestration layer through RESOURCE_PREPARERS, but concrete resource preparers still need to be implemented.

In particular, a callable for preparing psf_models (e.g. through the PSF inference pipeline) will be added in a follow-up PR once the appropriate inference API has been defined.