Tests | Additional test cases

### Summary of all missing cases

| # | Constructor | Input | What to assert |
|---|---|---|---|
| 1 | 1-arg `(string)` | `null` | `ApplicationClientId == SqlClientId`, `UseWamBroker == true` |
| 2 | 2-arg `(callback, string?)` | non-null callback + `SqlClientApplicationId` | `UseWamBroker == true` |
| 3 | 2-arg `(callback, string?)` | `null` callback + custom ID | No throw; `_deviceCodeFlowCallback == DefaultDeviceFlowCallback` |
| 4 | Options ctor | `ApplicationClientId = SqlClientId, UseWamBroker = true` | `UseWamBroker == true` |
| 5 | Any ctor with non-null callback | explicit callback | `_deviceCodeFlowCallback` is that exact instance |
| 6 | Any ctor without callback | omitted/null callback | `_deviceCodeFlowCallback` is `DefaultDeviceFlowCallback`, not null |
| — | Duplicate | `Ctor_ApplicationClientId_EnablesWamBroker` | Remove in favour of `Ctor_AppClientId_SqlClientId_EnablesWamBroker` |

_Originally posted by @paulmedynski in https://github.com/dotnet/SqlClient/pull/4288#discussion_r3435183576_

Locate the constructor tests and the existing Ctor_ApplicationClientId_EnablesWamBroker test. Add coverage for the listed null, custom-ID, callback, options, and default-callback cases, remove the duplicate test, and run the relevant test class. Done means every assertion in the table passes and the duplicate is gone.

Written by the indexing model from the issue text.