Have partialsnapshot use typeddicts instead of dicts.

[jonathan-eq]

Approach ✏️

(Screenshot of new behavior in GUI if applicable)

• [x] PR title captures the intent of the changes, and is fitting for release notes.
• [x] Added appropriate release note label
• [x] Commit history is consistent and clean, in line with the contribution guidelines.
• [ ] Make sure tests pass locally (after every commit!)

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.14815% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.20%. Comparing base (7440057) to head (fd221ba). Report is 5 commits behind head on main.

Files	Patch %	Lines
src/ert/ensemble_evaluator/snapshot.py	98.14%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #8122 +/- ## ========================================== + Coverage 86.08% 86.20% +0.12% ========================================== Files 383 383 Lines 23744 23784 +40 Branches 630 635 +5 ========================================== + Hits 20440 20504 +64 + Misses 3229 3207 -22 + Partials 75 73 -2 ```

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[jonathan-eq]

Will this PR be superseded by https://github.com/equinor/ert/pull/8182? @JHolba