fix(clinical): detect duplicate schedules by SQL error number

[rlorenzo]

What

In ScheduleEditService.AddInstructorAsync, the post-SaveChanges catch decides whether a DB failure means "instructor already scheduled" by SQL Server error number (2627 unique-constraint, 2601 duplicate-key-in-unique-index) instead of substring-matching the exception message.

private static bool IsDuplicateKeyViolation(Exception ex)
{
    var sqlEx = ex as SqlException ?? ex.InnerException as SqlException;
    return sqlEx is not null
        && sqlEx.Errors.Cast<SqlError>().Any(e => e.Number is 2627 or 2601);
}

Why

The old check matched "duplicate" / "unique" / "constraint" / "violation of primary key" against message.ToLower(). Two problems:

1. Latent bug: other constraint failures — foreign-key or check-constraint violations (error 547, whose message contains "constraint") — were mis-reported as "already scheduled." Now only true duplicate/unique violations get that message; everything else falls through to the generic "database operation failed" message.
2. Locale fragility: ToLower() plus English error-text matching is culture-sensitive (Turkish-I) and breaks if SQL messages are localized. Error numbers are stable and locale-invariant.

Notes

• Behavior-preserving for the real duplicate case; only re-routes non-duplicate constraint errors to the correct (generic) message.
• The duplicate pre-check (before SaveChanges) is unchanged; AddInstructorAsync_WithScheduleConflicts and all 2084 tests pass.
• Supersedes the message-matching helper currently on Development via refactor(codeql): decompose complex conditions + harden Vite web-root check #227; the overlap will be resolved (keeping this version) at the Development merge.

[codecov-commenter]

Bundle Report

Bundle size has no change ✅

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.48%. Comparing base (f6dbe72) to head (384a05b).

Files with missing lines	Patch %	Lines
.../ClinicalScheduler/Services/ScheduleEditService.cs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
- Coverage   44.49%   44.48%   -0.01%     
==========================================
  Files         895      895              
  Lines       51655    51654       -1     
  Branches     4812     4813       +1     
==========================================
- Hits        22983    22980       -3     
- Misses      28108    28110       +2     
  Partials      564      564              

Flag	Coverage Δ
backend	44.64% <0.00%> (-0.01%)	⬇️
frontend	41.32% <ø> (ø)

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[Copilot]

Pull request overview

Refines error handling in the Clinical Scheduler’s ScheduleEditService.AddInstructorAsync so that “already scheduled” is only returned for true SQL Server duplicate/unique key violations, using stable SQL error numbers instead of locale-dependent substring matching on exception messages.

Changes:

• Replaces exception message substring matching with SQL Server error-number detection (2627, 2601) to identify duplicate-key violations.
• Adds a focused helper (IsDuplicateKeyViolation) to unwrap EF/SQL exceptions and check SqlException.Errors.