[Task] Merge DataFileClass and OsemosysClass for v5.6, keep MUIOGO solver code

[SeaCelo]

Biggest piece of the v5.6 sync. Upstream extracted ~1,800 lines of DataFileClass into the new HelpersClass, and rewrote how OsemosysClass finds the GLPK/CBC solver on the user's machine. We need to absorb the helpers refactor without losing MUIOGO's solver-resolution work.

What's in scope:

• merge upstream's DataFileClass.py rewrite (use the Helpers extraction)
• merge upstream's OsemosysClass.py constructor refactor (Path-based, Helpers-based parameter building)
• keep MUIOGO's solver-resolution chain (env var → shutil.which → bundled). Adopting upstream's version wholesale would re-introduce bugs we already fixed in [Bug] Non-Windows solver paths in OsemosysClass are hardcoded dead-end stubs #43, [Bug] Windows .bat installation fails 🦇 #86, fix(windows): improve solver installation flow with CBC fallback (Refs #86) #101, Fix Windows GLPK fallback in setup #262
• borrow only: macOS Homebrew + Linux distro PATH probes, and the is_bundled-conditional cwd pattern in DataFileClass.run
• drop MUIOGO's recursive bundled-folder search (dead code today)
• drop upstream's dormant run_26022026 method (it contains shell=True calls; UPSTREAM_SYNC.md rejects)
• preserve Config.validate_path(Config.DATA_STORAGE, case) as the first line of OsemosysClass.__init__ (MUIOGO security fix from dc8f3c60, not present upstream)
• guard the Indicators.json / Duals.json reads in the constructor with try/except → empty dict
• use self.genData.get('osy-indicators', []) instead of unguarded subscript
• backend CaseRoute.py (/saveParamFile route): persist DualData and IndicatorData alongside ParamData and VarData

What's not in scope:

• frontend changes (handled separately)
• upload/config view (handled in the cosmetic sub-issue)

Done when:

• branch base: feature/v56-ui-sync
• demo case solves end-to-end with both GLPK and CBC
• on macOS: brew install glpk cbc works, app finds solvers
• on Linux: equivalent via apt
• on Windows: existing solver detection preserved

Notes:

• branch base: feature/v56-ui-sync (not main)
• depends on [Task] Port HelpersClass.py, Modals.html, and stub Indicators/Duals JSONs for v5.6 #458 (HelpersClass.py)
• upstream has resolve_solver_executable duplicated in both HelpersClass.py and OsemosysClass.py — we use neither, keep MUIOGO's local code

Related: #457, #43, #86, #101, #262, docs/UPSTREAM_SYNC.md