.Net: [.NET] Add TimeProvider injection to TimePlugin for deterministic testing

[grvnmttl]

Closes #14111

Summary

• Adds a TimeProvider constructor parameter to TimePlugin (defaults to TimeProvider.System, so all existing callers are unaffected)
• Replaces all direct DateTimeOffset.Now / DateTimeOffset.UtcNow calls with _timeProvider.GetLocalNow() / _timeProvider.GetUtcNow()
• Rewrites TimePluginTests from 5 real-clock tests to 26 deterministic tests covering every public method, using an inline FixedUtcTimeProvider subclass (no new test dependencies)

Key decisions

Decision	Reason
System.TimeProvider (not a custom IClock)	Standard .NET 8+ pattern; no extra abstraction needed
Inline FixedUtcTimeProvider subclass in tests	Avoids adding Microsoft.Extensions.TimeProvider.Testing as a new dependency; 3-line subclass is clearer
LocalTimeZone = TimeZoneInfo.Utc on the test provider	Makes test output identical on any machine regardless of OS timezone
Default TimeProvider.System in constructor	Zero breaking change — all existing callers (new TimePlugin()) work unchanged

Test results

Passed! - Failed: 0, Passed: 394, Skipped: 0, Total: 394

[Copilot]

Pull request overview

This PR makes TimePlugin deterministic-test friendly by introducing TimeProvider injection and updating all “current time” computations to use the injected provider rather than directly reading the system clock. It also rewrites the unit tests to use a fixed-time provider and validate formatted outputs deterministically.

Changes:

• Add TimeProvider support to TimePlugin and route time reads through it (GetLocalNow() / GetUtcNow()).
• Replace real-clock dependent tests with deterministic tests using an inline fixed TimeProvider implementation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
dotnet/src/Plugins/Plugins.Core/TimePlugin.cs	Introduces TimeProvider-based clock access and updates time/date formatting methods accordingly.
dotnet/src/Plugins/Plugins.UnitTests/Core/TimePluginTests.cs	Replaces non-deterministic tests with fixed-time, culture-stable assertions across plugin methods.

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[grvnmttl]

@microsoft-github-policy-service agree

[grvnmttl]

All four Copilot review items addressed in the latest commit:

1. Binary breaking change — kept the single optional-parameter constructor (TimeProvider? timeProvider = null). Adding a separate explicit parameterless overload would cause ActivatorUtilities (used by KernelPluginFactory.CreateFromType) to throw on ambiguity. Since Plugins.Core is a preview package with no stable release, there are no compiled consumer binaries to break; all existing new TimePlugin() call sites remain source-compatible.
2. netstandard2.0 compatibility — added Microsoft.Bcl.TimeProvider (8.0.1) to Directory.Packages.props and as a netstandard2.0-only PackageReference in Plugins.Core.csproj.
3. TimeZoneName() uses injected timezone — now returns _timeProvider.LocalTimeZone.DisplayName instead of TimeZoneInfo.Local.DisplayName.
4. TimeZoneName() test — added; asserts the provider's LocalTimeZone is reflected. Test count: 395, all passing.