MudDataGrid: Preserve initial sort definitions in single mode

[pttydou]

Fixes #9021.

MudDataGrid.SetParametersAsync compared the first incoming SortMode with the parameter's default value, so an initial Single mode was treated as a later mode transition and cleared caller-provided SortDefinitions. This change limits that clearing to post-first-render mode changes, preserving the existing behavior for real transitions.

Before/After:

• Before: the initial Name sort is ignored and rows render in input order (B, A, C).
• After: the same initial Name sort remains active and rows render as (A, B, C).

Tests:

• dotnet test --project src/MudBlazor.UnitTests/MudBlazor.UnitTests.csproj --no-restore /p:SkipBunCompile=true -- --filter "FullyQualifiedName~DataGridSortDefinitionsPreservedOnInitialSingleModeRender" --output Normal --no-ansi --hangdump --hangdump-timeout 120s
• dotnet test --project src/MudBlazor.UnitTests/MudBlazor.UnitTests.csproj --no-build --no-restore -- --filter "FullyQualifiedName~DataGridTests.DataGridSort" --output Normal --no-ansi --hangdump --hangdump-timeout 120s
• dotnet test --project src/MudBlazor.UnitTests/MudBlazor.UnitTests.csproj --no-build --no-restore -- --filter "FullyQualifiedName~DataGridCustomSort|FullyQualifiedName~DataGridFilteredItemsCache" --output Normal --no-ansi --hangdump --hangdump-timeout 120s
• dotnet format whitespace --no-restore --verify-no-changes --include MudBlazor/Components/DataGrid/MudDataGrid.razor.cs MudBlazor.UnitTests/Components/DataGridTests.cs

AI assistance was used through Codex with GPT-5.6 Sol for investigation, implementation, test drafting, and diff review. Human verification: before submission, the contributor confirmed that the attached screenshots from the local Viewer run show the same scenario changing from B, A, C to A, B, C.

Checklist:

• I've read the contribution guidelines
• My code follows the style of this project
• I've added or updated relevant unit tests

[versile2]

LGTM

[danielchalmers]

Thanks!