Standardize api

[Dev-Zully]

All related tests pass (79/79). Here's a summary of everything delivered for Issue closes #562 :

Summary of Changes
New Files
src/common/errors/error-envelope.ts — Shared ErrorEnvelope, ValidationErrorField, and ErrorCode enum. This is the single source of truth for the API error contract that clients can import for type safety.

src/common/errors/index.ts — Barrel export for the error types.

test/health.e2e-spec.ts — 18 e2e tests covering all error contract acceptance criteria:

Envelope shape contract (success=false, code, message)
Business error contract (domain codes preserved)
404 error contract
Unhandled exception contract (production vs dev sanitization)
Correlation ID contract (echo, auto-generate, consistency)
Rate-limit and domain error shape contracts (static documentation)
Response headers contract
Modified Files
src/common/filters/global-http-exception.filter.ts — Replaced inline ErrorResponseBody type with shared ErrorEnvelope. Uses ErrorCode enum for stable codes. Re-exports types for downstream consumers.

src/common/utils/validation-error.mapper.ts — Uses shared ValidationErrorField type.

src/main.ts — Uses ErrorCode.VALIDATION_ERROR enum instead of raw string.

test/smoke.e2e-spec.ts and test/app.e2e-spec.ts — Updated to use ErrorCode enum.

jest.config.ts and jest.e2e.config.ts — Fixed moduleFileExtensions order (ts before js) to prevent stale .js build artifacts from shadowing .ts sources in test resolution.

Verification
✅ TypeScript: Zero new type errors (all 51 errors are pre-existing in unrelated modules)
✅ E2E tests: 18/18 pass in health.e2e-spec.ts
✅ Unit tests: 889 pass (29 failures all pre-existing in unrelated modules)
✅ CI-compatible: All changes follow existing conventions and test patterns

[MaryammAli]

LGTM
a job well done Dev
this issue was implemented in a short period of time
keep up the good work