[Tizen.WindowSystem] Replace manual null checks with ArgumentNullException.ThrowIfNull()

[JoonghyunCho]

Summary

Replaces legacy if (arg == null) throw new ArgumentNullException(nameof(arg)); patterns in src/Tizen.WindowSystem with the single-statement helper ArgumentNullException.ThrowIfNull(arg) introduced in .NET 6.

This is the second module-level slice of the umbrella refactor tracked in #7548 (the first slice, Tizen.Applications.Common, is in PR #7566).

Changes

• src/Tizen.WindowSystem/src/KVMService.cs — constructor null checks for tzShell, win
• src/Tizen.WindowSystem/src/QuickPanelService.cs — constructor null checks for tzShell, win; SetContentRegion and SetHandlerRegion null check for regions
• src/Tizen.WindowSystem/src/QuickPanelClient.cs — constructor null checks for tzShell, win (also normalizes the pre-existing literal-string paramNames "tzShell" / "win" — ArgumentNullException.ThrowIfNull derives the same paramName via CallerArgumentExpression)
• src/Tizen.WindowSystem/src/ScreensaverService.cs — constructor null checks for tzShell, win
• src/Tizen.WindowSystem/src/TaskbarService.cs — constructor null checks for tzShell, win
• src/Tizen.WindowSystem/src/SoftkeyClient.cs — constructor null checks for tzShell, win
• src/Tizen.WindowSystem/src/SoftkeyService.cs — constructor null checks for tzShell, win

Net change: 7 files, +16 / -64 lines.

Mode

Refactoring

API Compatibility

Item	Result
Public API signatures	Unchanged
Thrown exception type	Unchanged (ArgumentNullException)
paramName carried by exception	Unchanged (same nameof(arg) value; for the previous literal-string sites in QuickPanelClient.cs, the literal already matched the parameter name)
Native interop surface	Unchanged
Backward compatibility	100% preserved

Verification

• Build: dotnet build src/Tizen.WindowSystem → 0 errors, 4 unrelated warnings inherited from Tizen.Applications.Common
• Tests: N/A — no behavior change; existing unit tests in dependent suites remain valid
• Benchmark: skipped — the change is a behavior-preserving IL-size reduction; per-call cost is identical and these are not on hot paths. No sdb device available in this environment for runtime measurement.

Refs #7548

[gemini-code-assist]

Code Review

This pull request refactors several service and client classes within the Tizen.WindowSystem namespace to use the ArgumentNullException.ThrowIfNull helper method for parameter validation, replacing manual null checks with a more concise implementation. I have no feedback to provide as there were no review comments to assess.

[JoonghyunCho]

🤖 [AI Review]

Reviewed — no findings.

Scope checked:

• TFM compatibility: Tizen.WindowSystem.csproj targets net8.0; ArgumentNullException.ThrowIfNull is available since .NET 6 ✓
• 11 replacement sites across 7 files — all are vanilla if (x == null) throw new ArgumentNullException(...) blocks with no custom message (no information lost)
• Mixed legacy patterns handled: both nameof(x) and string-literal "x" forms (e.g., QuickPanelClient) consolidated — ThrowIfNull's [CallerArgumentExpression] produces the correct parameter name in either case
• Surrounding ArgumentException checks (tzShell.SafeHandle == null || IsInvalid) intentionally left untouched — semantics differ from ArgumentNullException
• Public API surface unchanged: signatures, exception types, and <exception cref="ArgumentNullException"> doc tags remain accurate

No 🔴 critical issues, no 🟡 suggestions to flag.

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Automated review — final merge decision rests with human reviewers.