Fix missing Microsoft.Extensions.Configuration using in generated ClientSettings when BindCore is customized - #10905

…entSettings when BindCore is customized (microsoft#10905)

When a generated `ClientSettings` type's
`BindCore(IConfigurationSection)` is moved to custom code, the generated
partial still references `IConfigurationSection` in its XML doc summary,
but `using Microsoft.Extensions.Configuration;` was dropped — so the
generated code no longer compiles.

**Root cause:** Namespace imports are registered from actual type
references during writing. The class description used a plain string
`<see cref="IConfigurationSection"/>` rather than a real type reference,
so it never registered the namespace. The only thing registering
`Microsoft.Extensions.Configuration` was the generated `BindCore`
parameter type — once `BindCore` is customized away and filtered out,
nothing registers it.

### Changes
- **`ClientSettingsProvider.BuildDescription`**: reference the actual
types via the `:C` cref formatter (matching the `ClientOptionsProvider`
convention) instead of hardcoded `<see cref>` strings. This registers
the `Microsoft.Extensions.Configuration` namespace while rendering an
identical cref, so normal generated output is unchanged.

```csharp
// before
=> $"Represents the settings used to configure a <see cref=\"{_clientProvider.Name}\"/> that can be loaded from an <see cref=\"IConfigurationSection\"/>.";
// after
=> $"Represents the settings used to configure a {_clientProvider.Type:C} that can be loaded from an {IConfigurationSectionType:C}.";
```

- **Regression test** (`TestGeneratedSettings_WithCustomizedBindCore`) +
TestData: uses TestData to move `BindCore` into a custom partial,
asserts it is no longer generated, and validates the full generated
output against an expected snapshot file
(`TestData/ClientSettingsProviderTests/TestGeneratedSettings_WithCustomizedBindCore.cs`),
confirming the `using` is still emitted. Fails without the fix.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>