test: stabilize reminder and activation collection tests

[ReubenBond]

Two Functional tests depended on startup and collector timing, which made them vulnerable to slow CI runs. This changes them to wait for observable readiness or convergence instead of relying on fixed delays.

The minimal reminder test now retries only while the reminder service reports that it is still initializing, waits until the registered reminder is readable, and then stops it. The activation collection test now triggers the same stale-activation collection path used by the collector timer through an internal test hook, verifies the default collection age does not collect the target grain type, and waits for the target activation count to converge to zero under the class-specific age limit.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR stabilizes two Functional tests which were previously sensitive to CI timing by replacing fixed delays with readiness/convergence-based waiting and by adding an internal test hook to deterministically trigger the same activation-collection path used by the runtime’s collector timer.

Changes:

• Updates the activation-collection functional test to force stale-activation scans via a new internal ActivationCollector helper and to wait for activation-count convergence instead of sleeping a fixed duration.
• Updates the minimal reminder functional test to retry reminder operations only while the reminder service is initializing and to wait until a created reminder is readable before stopping it.
• Adds an internal ActivationCollector.CollectStaleActivations method to expose the collector’s stale-scan path for internal tests.

Show a summary per file

File	Description
test/Orleans.Runtime.Internal.Tests/ActivationsLifeCycleTests/ActivationCollectorTests.cs	Reworks the class-specific collection-age test to use deterministic stale-collection triggering and convergence waiting.
test/Orleans.Reminders.Tests/MinimalReminderTests.cs	Adds initialization-aware retry loops around reminder start/read/stop to avoid fixed timing assumptions.
src/Orleans.Runtime/Catalog/ActivationCollector.cs	Introduces an internal method to invoke the stale activation scan path used by the collection timer.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2