Non optional everest model and nonzero realizations

[StephanDeHoop]

Issue
Resolves #9368

Approach
Make ModelConfig required and enforce realizations length > 0. Add some more meaningful error messages too. I am still using list[NonNegativeInt] (with min_length=1 in Field) instead of conlist since this was ill-advised (see discussion here microsoft/pylance-release#5457).

NOTE: If I don't add if values is None: we get this error when model is specified but left empty: 'NoneType' object has no attribute 'get' which doesn't tell the user anything :)

(Screenshot of new behavior in GUI if applicable)

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codspeed-hq]

CodSpeed Performance Report

Merging #9577 will degrade performances by 11.56%

_{Comparing StephanDeHoop:non_optional_everest_model_and_nonzero_realizations (a2ffbdc) with main (bc8be8f)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	StephanDeHoop:non_optional_everest_model_and_nonzero_realizations	Change
❌	test_load_from_context[gen_x: 20, sum_x: 20 reals: 10]	5.8 ms	6.6 ms	-11.56%