fix: batch errors are now rendered

[raizasafeel]

Summary

• Creating a batch with missing required fields failed silently: frontend toast was blank, backend crashed before the mandatory check ran.

Changes

• lms_batch.py: run _validate_mandatory() first
• NewBatchModal.vue: fall back to a generic toast when err.messages is empty

[cypress]

lms    Run #3669

Run Properties:   Passed #3669  •  2d2b421784 ℹ️: Merge 9e4f1d831f62363c18e59df7626cf0e7493312f7 into 41fb007cc7d039190c6b141af896...

Project	lms
Branch Review	fix/batch-creation
Run status	Passed #3669
Run duration	00m 26s
Commit	2d2b421784 ℹ️: Merge 9e4f1d831f62363c18e59df7626cf0e7493312f7 into 41fb007cc7d039190c6b141af896...
Committer	Raizaaa
View all properties for this run ↗︎

---

Test results
Failures	0
Flaky	0
Pending	0
Skipped	0
Passing	1
View all changes introduced in this branch ↗︎

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.76%. Comparing base (41fb007) to head (9e4f1d8).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2407      +/-   ##
===========================================
+ Coverage    47.74%   47.76%   +0.02%     
===========================================
  Files          150      150              
  Lines         6658     6659       +1     
===========================================
+ Hits          3179     3181       +2     
+ Misses        3479     3478       -1     

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