feat: auto-infer [CombinedDataSources] for single-parameter methods

[thomhurst]

Problem

When a test method has a single parameter with a data source attribute, [CombinedDataSources] is still required:

[Test]
[CombinedDataSources] // seems redundant for a single parameter
public async Task Unauthenticated_Requests_Are_Rejected(
    [ClassDataSource<ApiClientFixture>(Shared = SharedType.PerTestSession)]
    ApiClientFixture client)
{
    var response = await client.Client.GetAsync("/api/orders/mine");
    await Assert.That(response.StatusCode).IsEqualTo(HttpStatusCode.Unauthorized);
}

Without it, you get TUnit0070 and TUnit0038 errors.

Proposed Behavior

When a method has exactly one parameter with a data source attribute, TUnit could automatically treat it as a combined data source (there's nothing to combine - it's just a single source). The [CombinedDataSources] attribute would only be strictly required when there are 2+ parameters with data source attributes.

Alternatively, this could be its own attribute concept - something like parameter-level [ClassDataSource] being allowed without [CombinedDataSources] when there's only one parameter.

Context

Discovered while building the CloudShop Aspire + TUnit example (#4761). The AuthorizationTests class had a simple single-parameter test that required the extra attribute.

[thomhurst]

Closing - TUnit's design favors explicit attribute usage. [CombinedDataSources] should remain required even for single-parameter methods to maintain consistency and clarity.