refactor(runtime): start grain activation synchronously

[ReubenBond]

Follow-up to #10565 and completion of the fix for #10556. This PR builds on the grain-context configuration contract merged in #10565.

IGrainContext now exposes a one-shot startup contract directly: Start() returns a release token and Abort() cleans up an unstarted context. Built-in activation creation configures the context with scheduler execution reserved, publishes or wires it, then calls Start() to construct the grain exactly once under activation scheduler ownership. Rehydration and activation operations are queued before disposing the token releases the prequeued message loop. Failures after construction use normal deactivation and instance disposal.

The construction task is created under a clean ExecutionContext so ambient request and transaction AsyncLocal values do not leak into grain construction. DefaultExecutionContext captures a clean context using ThreadPool.UnsafeQueueUserWorkItem, failing fast if the work item cannot be queued.

Custom eager contexts retain compatibility through the default no-op Start()/Abort() implementations. Focused regression coverage verifies exactly-once startup, deferred work, abort/reuse, scheduler and runtime-context affinity across asynchronous signals, request and transaction ambient-state isolation, hosted-client streams, duplicate-activation stress, stateless-worker constructor failure, activation tracing, and direct activator behavior on both net8.0 and net10.0.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR refactors Orleans runtime grain activation startup so that activation begins synchronously on the activation scheduler before GrainContextActivator.CreateInstance returns, while ensuring the transition runs under a clean ExecutionContext to prevent AsyncLocal/ambient state leakage into grain construction/activation.

Changes:

• Start activation synchronously on the activation scheduler using Task.RunSynchronously, explicitly installing the activation as the current RuntimeContext.
• Introduce DefaultExecutionContext to obtain an ambient-state-free ExecutionContext (CoreCLR fast-path via UnsafeAccessor, with a portable fallback).
• Add focused tests covering scheduler affinity + ambient isolation during activation and validating DefaultExecutionContext behavior (including concurrent use).

Show a summary per file

File	Description
test/Orleans.Runtime.Tests/GrainActivatorTests.cs	Adds a regression test ensuring synchronous activation start has scheduler affinity, correct RuntimeContext, and no RequestContext/transaction ambient leakage at construction time.
test/Orleans.Core.Tests/Runtime/DefaultExecutionContextTests.cs	Adds unit tests validating DefaultExecutionContext contains no ambient state and is safe under concurrent ExecutionContext.Run usage.
src/Orleans.Runtime/Utils/DefaultExecutionContext.cs	Adds a utility to obtain an ambient-free ExecutionContext via runtime default or a suppressed-flow capture fallback.
src/Orleans.Runtime/Activation/ActivationDataActivatorProvider.cs	Switches activation startup to ExecutionContext.Run(DefaultExecutionContext.Instance, ...) and runs activation start synchronously on the activation task scheduler.

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• Files reviewed: 5/5 changed files
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Copilot review overview

Review tier: Lite
Findings: 2 · 1

Open findings (3)

• Inside Start(), the constructor execution uses context._startup! rather than the… · New
• This change removes the activation-timeout cancellation token previously used during… · New
• This assertion indexes measurement.Tags[\"type\"] directly. If any collected measurement lacks… · New