Add exception handler and tests for duplicate budget constraint

[Copilot]

The API lacked proper HTTP error handling for duplicate budget attempts, falling back to generic 422 errors instead of returning semantic 409 Conflict responses.

Changes

• Exception mapping: Added BudgetAlreadyExistsException case to GlobalExceptionMapper returning 409 with BUDGET_ALREADY_EXISTS error code
• Unit test: Added shouldThrowExceptionWhenBudgetAlreadyExists verifying domain exception is thrown when budget exists for current month
• Integration test: Added shouldRejectDuplicateBudgetForSameMonth verifying full API flow returns 409 on duplicate attempts

API Response

POST /api/companies/{id}/budget
// Second call in same month returns:
{
  "error": {
    "code": "BUDGET_ALREADY_EXISTS",
    "message": "A budget already exists for this company for the current month",
    "traceId": "..."
  }
}

The duplicate detection logic (findByCompanyIdAndEffectiveFrom check) was already present from commit 848eb2d.

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[Copilot]

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[Copilot]

Pull request overview

Adds explicit REST-layer handling for duplicate monthly company budget creation so the API returns a semantic 409 Conflict (with a specific error code) instead of the generic domain fallback 422.

Changes:

• Added BudgetAlreadyExistsException handling to GlobalExceptionMapper, returning HTTP 409 with BUDGET_ALREADY_EXISTS.
• Added a use-case unit test asserting BudgetAlreadyExistsException is thrown and no persistence/audit occurs.
• Added an integration test asserting the second budget POST in the same month is rejected with 409 and the expected error code.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
apps/api/src/main/java/com/upkeep/infrastructure/adapter/in/rest/common/exception/GlobalExceptionMapper.java	Maps BudgetAlreadyExistsException to HTTP 409 and emits BUDGET_ALREADY_EXISTS error code.
apps/api/src/test/java/com/upkeep/application/usecase/SetCompanyBudgetUseCaseImplTest.java	Unit test coverage for duplicate budget detection at the use-case level.
apps/api/src/test/java/com/upkeep/infrastructure/adapter/in/rest/budget/BudgetResourceTest.java	Integration test coverage ensuring the API returns 409 on duplicate monthly budget creation.

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