docs: resolve CS1574 unresolved crefs and CS1571 duplicate param in TodoApp.BlazorWasm.Client TodoService

[adrianhall]

Summary

Fixes the CS1574 unresolved XML doc cref warnings and the CS1571 duplicate <param> warning reported in #550, surfaced when building TodoApp.BlazorWasm.Client with -p:GenerateDocumentationFile=true.

Traced each cref against the actual CommunityToolkit.Datasync.Client source rather than guessing:

• ValidationException / ConcurrencyException don't exist in CommunityToolkit.Datasync.Client, and are never actually thrown by AddAsync/ReplaceAsync today — a rejected/invalid write surfaces as ConflictException<T> (409/412) or EntityDoesNotExistException (404, since DatasyncServiceOptions.ThrowIfMissing defaults to true), thrown by the client library itself. Updated the exception docs on CreateTodoItemAsync/UpdateTodoItemAsync/DeleteTodoItemAsync in both ITodoService.cs and TodoService.cs to reference these real types instead.
• DatasyncServiceClient{T}.AddAsync(T) / .ReplaceAsync(T) don't exist with those signatures — the code actually calls the IDatasyncServiceClientExtensions.AddAsync/ReplaceAsync extension methods (2-arg: entity + CancellationToken), so the crefs now target those, mirroring the fix pattern already used for EntityFrameworkQueryableExtensions.AnyAsync in docs: fix CS1574 XML cref for AnyAsync #553.
• DatasyncServiceClient{T}.RemoveAsync(string, DatasyncServiceOptions) was just missing its CancellationToken parameter.
• Merged the duplicate <param name="title"> tag on CreateTodoItemAsync in both files into a single tag positioned after <summary>.

Verification

• dotnet build samples/todoapp-blazor-wasm/TodoApp.BlazorWasm.Server/TodoApp.BlazorWasm.Server.csproj -p:GenerateDocumentationFile=true — 0 errors, 0 CS1574/CS1571 warnings (previously 6). One pre-existing, unrelated WASM native-linking warning remains.
• dotnet restore / dotnet build on Datasync.Toolkit.sln — succeeds, 0 warnings, 0 errors.
• dotnet test tests/CommunityToolkit.Datasync.Client.Test — 1432/1432 passed (the test project most relevant to the types referenced in this change).
• Full Build Samples GitHub Actions workflow dispatched against this branch on my fork — all 17 jobs + aggregation check passed: https://github.com/adrianhall/CommunityToolkit-Datasync/actions/runs/29145912239

Full solution dotnet test was not run end-to-end: several test projects (Cosmos, MongoDB, SQL Server, LiteDb) require live database services not available in this environment, and are unrelated to this docs-only sample change (no files under src/ or tests/ were touched).

Fixes #550