.NET: Fix and re-enable flaky InputWaiter timeout test

[peibekwe]

Motivation & Context

InputWaiterTests.InputWaiter_WaitForInputAsync_CompletesWhenTimeoutExpiresAsync was quarantined in #7361 because it failed intermittently on the net472/windows-latest leg of merge_group runs, blocking the merge queue for PRs that never touched workflow code.

The test raced a 300ms wait against a 5s Task.Delay and then asserted which one won by comparing task identity. That assertion measured scheduling latency rather than behaviour: SemaphoreSlim.WaitAsync(timeout) reaches its timeout through several thread-pool hops, while Task.Delay completes directly in its timer callback. On a loaded runner the semaphore honored its 300ms timeout exactly, but the continuation was not observed until after the 5s guard, so the delay won and the test failed.

Re-enable the quarantined test and also fix related siblings. Fixes #7360

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

This PR addresses a previously quarantined flaky workflows unit test by removing a wall-clock race and asserting timeout behavior via the API result instead of task scheduling/identity, allowing the test to be re-enabled without destabilizing net472 CI legs.

Changes:

• Introduces a consistent “guard timeout” backstop in InputWaiterTests to prevent hangs while keeping assertions focused on behavior.
• Rewrites the timeout test (and related siblings) to assert on a boolean result (signaled vs timed out) rather than racing SemaphoreSlim.WaitAsync(timeout) against Task.Delay.
• Updates InputWaiter.WaitForInputAsync(TimeSpan, ...) to return bool, exposing whether the wait ended due to a signal or a timeout.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/InputWaiterTests.cs	Re-enables and stabilizes the timeout test by asserting on a returned flag and adding a liveness guard.
dotnet/src/Microsoft.Agents.AI.Workflows/Execution/InputWaiter.cs	Returns a boolean from the timeout wait overload to distinguish signal vs timeout and support non-flaky tests.

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 94% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

---

Automated review by peibekwe's agents

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 94% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

---

Automated review by peibekwe's agents