Add tests for auto-store-data.js middleware (#81)

Adds test coverage for the `auto-store-data.js` Express middleware using
Node's native test and assert libraries.

## Test Coverage

- **Session management**: Initialization and preservation of
`req.session.data`
- **Data sources**: POST body and GET query parameters (query overrides
body)
- **Field filtering**: Underscore-prefixed fields are ignored
- **Checkbox handling**: `_unchecked` values delete fields; filtered
from arrays
- **Nested objects**: Recursive merging and object initialization
- **Response locals**: Population of `res.locals.data` from session

## Example Test Pattern

```javascript
describe('autoStoreData middleware', () => {
  let req, res, nextCalled

  beforeEach(() => {
    req = { body: {}, query: {}, session: {} }
    res = { locals: {} }
    nextCalled = false
  })

  const next = () => { nextCalled = true }

  it('should filter _unchecked from array values', () => {
    req.body = { preferences: ['email', '_unchecked', 'sms'] }
    autoStoreData(req, res, next)
    assert.deepStrictEqual(req.session.data.preferences, ['email', 'sms'])
  })
})
```

26 test cases added following the existing test patterns in
`tests/express-middleware/`.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Add tests for `auto-store-data.js`</issue_title>
> <issue_description>These tests should follow the pattern of the
existing ones, and use the Node assert and test
libraries.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #80

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: frankieroberto <30665+frankieroberto@users.noreply.github.com>

Author: Copilot