Enhance PUT /Expense handling for partial updates, add validation and tests

[linjinhsien]

Motivation

• Prevent overwriting existing expense fields on PUT when clients send partial payloads and to centralize validation logic for updates.
• Provide clearer logging and proper NotFound/BadRequest responses for invalid or missing resources.
• Add unit tests to cover partial updates and mismatched id behavior.

Description

• Replace direct Entry(...).State = EntityState.Modified with loading the existing entity via FindAsync(id) and selectively applying non-default fields from the incoming Expense object.
• Add validations on update to enforce non-empty Description, disallow negative Amount, and enforce the existing 午餐 amount cap, returning appropriate BadRequest messages and logging warnings.
• Return NotFound when the expense to update does not exist and return BadRequest when route and body ids mismatch (unless body id is zero).
• Improve logging for create, update, and delete flows to include contextual ids.
• Add two unit tests: PutExpense_PartialUpdateWithoutDate_KeepsOriginalDate and PutExpense_MismatchedId_ReturnsBadRequest to validate partial update semantics and id mismatch handling.

Testing

• Ran the test suite in ExpenseApiTest, and the new tests PutExpense_PartialUpdateWithoutDate_KeepsOriginalDate and PutExpense_MismatchedId_ReturnsBadRequest passed.
• Existing tests including PostExpense_BoundaryExpense_ReturnsCreatedExpense were executed and passed.
• No automated tests failed after the changes.

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Codex Task

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[gemini-code-assist]

Code Review

This pull request updates the PutExpense endpoint in ExpenseController to support partial updates by retrieving the existing record before applying changes. It also introduces validation rules for descriptions, lunch expense limits, and negative amounts, alongside new unit tests. Review feedback points out that Amount, Category, and Title are still being overwritten even when omitted from the request, which contradicts the partial update goal. It is also suggested to use constants for hardcoded strings and to expand test assertions to verify that unmodified fields are preserved.

[gemini-code-assist]

These fields are currently overwritten regardless of whether they were provided in the request body. This contradicts the PR's objective of supporting partial updates. If a client omits these fields, they will default to 0 or null, causing existing data to be lost in the database.

            if (expense.Amount != 0)
            {
                existingExpense.Amount = expense.Amount;
            }

            if (expense.Category is not null)
            {
                existingExpense.Category = expense.Category;
            }

            if (expense.Title is not null)
            {
                existingExpense.Title = expense.Title;
            }

[gemini-code-assist]

The string "午餐" is hardcoded here and in the PostExpense method. It is recommended to define this as a constant to improve maintainability and prevent potential bugs from typos.

[gemini-code-assist]

To properly verify the partial update logic, this test should also assert that fields not included in the updateRequest (such as Category and Title) retain their original values. This would help detect the current issue where these fields are being overwritten with null.

        Assert.Equal(120, updatedExpense.Amount);
        Assert.Equal("食", updatedExpense.Category);
        Assert.Equal("早餐", updatedExpense.Title);