fix: gracefully handle missing active session on download routes (fixes #415)

[tejassinghbhati]

Linked issue

• Closes [Bug] Unhandled TypeError (500 crash) in download endpoints when active case session is missing #415
• Related [Bug] Dead exception handlers in DataFileClass.run() due to incorrect exception order #400, fix: preserve original exception type in FileClass write methods #411

Existing related work reviewed

• Issues/PRs reviewed: [v5.5] Backend/runtime port: safe routes, logging, resData fixes #407, [Bug] Dead exception handlers in DataFileClass.run() due to incorrect exception order #400, fix: preserve original exception type in FileClass write methods #411
• If none found, write: None found after search

Overlap assessment

• Classification: Bug Fix / Edge Case Logic Fix
• Overlapping items: PR [v5.5] Backend/runtime port: safe routes, logging, resData fixes #407 touches missing resData handling, but entirely misses the missing session vulnerabilities in the download endpoints.
• Why this is not duplicate/superseded: No other PR addresses the missing session checks and TypeError crashes specifically occurring across the file download routes.

Why this PR should proceed

• It addresses a critical crash (500 Internal Server Error) that occurs when any of the 5 download routes are accessed without a correctly initialized active session, safely catching the error and returning an appropriate 400 response.

Summary

• What changed: In API/Routes/DataFile/DataFileRoute.py (4 routes) and API/Routes/Case/CaseRoute.py (1 route), added conditional block if case is None: returning a graceful 400 JSON response instead of blindly continuing to the dataset lookup.
• Why: The pathlib.Path constructor requires a string and cannot process a returned None from session.get('osycase', None). Doing so throws an unhandled TypeError in the route's primary thread, crashing the Flask route.

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

[SeaCelo]

@tejassinghbhati and @krrishrastogi05 thank you for working on this issue. I'm merging this PR, with a few additional fixes.