fix: preserve original exception type in FileClass write methods

[Anirudh77715]

This fixes an issue where IOError exceptions were swallowed and incorrectly re-raised as IndexError in writeFile and writeFileUJson.

Linked issue

• Closes [Bug] FileClass.py writeFile() swallows IOError and re-raises it as IndexError — misleading exception type masks real I/O failures #353

Existing related work reviewed

• Issues/PRs reviewed: [Bug] FileClass.py writeFile() swallows IOError and re-raises it as IndexError — misleading exception type masks real I/O failures #353 (bug report)
• No existing PR addresses this issue — None found after search

Overlap assessment

• Classification: none
• Overlapping items: none
• Why this is not duplicate/superseded: no other PR touches FileClass.py exception handling

Why this PR should proceed

Both writeFile() and writeFileUJson() catch (IOError, IndexError) but unconditionally re-raise as IndexError, masking the original exception type. When an IOError occurs during file write, callers see IndexError instead — making debugging misleading. Using bare raise preserves the original exception type and traceback.

Summary

• What changed: Replaced raise IndexError with bare raise in both writeFile() and writeFileUJson() in API/Classes/Base/FileClass.py
• Why: The original code masks IOError as IndexError, making debugging misleading; bare raise preserves the correct exception type

Validation

• Tests added/updated (or not applicable)
• Validation steps documented
• Evidence attached (logs/screenshots/output as relevant)

Documentation

• Docs updated in this PR (or not applicable)
• Any setup/workflow changes reflected in repo docs

Scope check

• No unrelated refactors
• Implemented from a feature branch
• Change is deliverable without upstream OSeMOSYS/MUIO dependency
• Base repo/branch is EAPD-DRB/MUIOGO:main (not upstream)

Exception rationale