[Hotfix Main]: Added result model for ForceOutput

[github-actions]

Hotfix of PR #1752 (#1752) to the main branch.
Hey @piotrkluba, please review this hotfix PR created from your original PR.

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Note

Medium Risk
Touches core results discovery/download plumbing and adds new file-pattern based behavior, which could affect what gets downloaded for cases depending on their params and available files.

Overview
Adds first-class support for ForceOutput-generated CSVs as a case result.

CaseResultsModel now exposes custom_forces backed by a new CustomForceResultModel that discovers result files matching force_output_(.+)_v2.csv, supports name-based access ([]/get_custom_force_by_name), and is wired into CaseResultsModel.download() and _is_downloadable via Case.has_custom_forces()/SimulationParams.has_custom_forces().

Also refactors monitor/UDD discovery into private _populate_* helpers, and extends tests + mock fixture data with force output CSVs and coverage for discovery, downloading hooks, and CSV loading.

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