test : added unit tests for server.utils.AppError class hierarchy

[tmdeveloper007]

Closes #1466.

Summary of What Has Been Done:
Added vitest tests for AppError and all subclasses (ValidationError, UnauthorizedError, ForbiddenError, NotFoundError, ConflictError) in server/utils/AppError.ts. Tests verify correct HTTP status codes, isOperational defaults, errorCode fields, instanceof checks, and stack trace capture. All 32 tests pass.

Changes Made:

• server/utils/AppError.test.ts (new file, 183 lines) — 32 test cases

Impact it Made:

• Ensures typed error classes reliably emit the correct HTTP status codes for Express error middleware
• Guards against silent regressions when modifying the error class hierarchy
• All tests green: npx vitest run server/utils/AppError.test.ts (32 passed)

Note: Please assign this PR to the tmdeveloper007 account.

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