fix(testing): make wait deadline tests deterministic

[ReubenBond]

Closes #10728

The reported failure occurred because the 25 ms test budget could expire before the first predicate invocation under CI load, so the test sometimes skipped the predicate-crossing path it intended to verify.

This change routes the helper's monotonic timestamps, deadline cancellation, and retry delays through an internal TimeProvider seam while production continues to use TimeProvider.System. The deadline tests use FakeTimeProvider plus explicit predicate barriers to cover completion immediately before the deadline, completion at the deadline, legacy predicate settlement, and deadline-token propagation.

The resulting invariant is deterministic: a predicate completion succeeds only while elapsed monotonic time is strictly less than the configured timeout.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a flaky deadline-boundary test in Orleans’ TestingUtils timing helpers by making the deadline logic testable with a deterministic clock, while keeping production behavior on TimeProvider.System.

Changes:

• Introduces an internal TimeProvider seam in TestingUtils.WaitUntilAsync/WaitUntilSucceededAsync to drive monotonic timestamps, deadline cancellation, and retry delays.
• Rewrites the deadline-focused tests to use FakeTimeProvider plus explicit predicate barriers to deterministically validate “success only when elapsed < timeout”.
• Grants the test assembly access to the new internal overloads via InternalsVisibleTo and adds the required testing package reference.

Show a summary per file

File	Description
test/TestInfrastructure/Orleans.TestingHost.Tests/TestingUtilsTests.cs	Reworks deadline tests to use FakeTimeProvider and explicit barriers for deterministic boundary coverage.
test/TestInfrastructure/Orleans.TestingHost.Tests/Orleans.TestingHost.Tests.csproj	Adds Microsoft.Extensions.TimeProvider.Testing dependency for FakeTimeProvider.
src/Orleans.TestingHost/Utils/TestingUtils.cs	Routes elapsed-time measurement, deadline CTS, and retry delays through an internal TimeProvider parameter (defaulting to TimeProvider.System).
src/Orleans.TestingHost/Orleans.TestingHost.csproj	Adds InternalsVisibleTo for Orleans.TestingHost.Tests so tests can call internal overloads.

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