CodeQL 13: chore: additional style fixes (ternary, complex-block, implicit-conversion)

[rlorenzo]

Summary

Final sweep of CodeQL style alerts that earlier PRs left as wontfix:

• js/implicit-operand-conversion (2): course-import-dialog.test.ts:72,96 - narrow the union check by combining typeof === 'object' with instanceof Error, then explicit as Error cast on the truthy branch.
• cs/complex-block (1): MeritMultiYearService.CalculateMedian - five for-loops appending to a List collapsed into chained Enumerable.Repeat(...).Concat(...).
• cs/missed-ternary-operator (5) that were deferred:
  • GradYearClassLevel.cs:63 - case-4 if-else assignment
  • RotationsController.cs:63 - serviceId.HasValue ? GetRotationsByServiceAsync : GetRotationsAsync
  • UinformService.cs:247 - response.IsSuccessStatusCode ? Deserialize : new ErrorResponse
  • EffortAuditService.cs:605 - termCode-keyed IQueryable selection
  • Also picks up the earlier useless-tostring fix on GradYearClassLevel.cs:57,60,69 (no-op vs PR CodeQL 12: chore: style sweep for remaining CodeQL alerts #198 which has the same change).

Context

Thirteenth in the CodeQL N: cleanup series.

Test plan

• npm run test:backend - 1946 tests passing
• Pre-commit lint+test+verify all passed

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[codecov-commenter]

Bundle Report

Bundle size has no change ✅

[codecov-commenter]

Codecov Report

❌ Patch coverage is 13.63636% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.97%. Comparing base (38de1ad) to head (8fc03de).

Files with missing lines	Patch %	Lines
web/Areas/Effort/Services/MeritMultiYearService.cs	0.00%	7 Missing ⚠️
web/Areas/Students/Services/GradYearClassLevel.cs	0.00%	5 Missing ⚠️
web/Areas/Effort/Services/EffortAuditService.cs	0.00%	4 Missing ⚠️
web/Areas/RAPS/Services/UinformService.cs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #199   +/-   ##
=======================================
  Coverage   42.96%   42.97%           
=======================================
  Files         877      877           
  Lines       51468    51450   -18     
  Branches     4802     4797    -5     
=======================================
- Hits        22113    22109    -4     
+ Misses      28831    28817   -14     
  Partials      524      524           

Flag	Coverage Δ
backend	43.05% <13.63%> (+<0.01%)	⬆️
frontend	41.34% <ø> (ø)

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