fix(runtime): preserve ReadOnly scheduling isolation

[ReubenBond]

Problem

When multiple [ReadOnly] requests were active, request admission relied on a single _blockingRequest. If that representative completed first, a writable request could start while another read-only request was still active. A single representative was also insufficient for heterogeneous MayInterleave cohorts.

Closes #10807.

Solution

• Evaluate heterogeneous admissions against every active request so each pair must be compatible.
• Preserve constant-time paths for ordinary and homogeneous read-only workloads using active-request counters.
• Keep _blockingRequest focused on diagnostics and stuck-request tracking by pointing it at the request which actually prevents admission.
• Add deterministic coverage for request flags, interface inheritance and redeclaration, overloads, generics, read/write ordering, both reader completion orders, AlwaysInterleave, Reentrant, and mixed MayInterleave survivors.
• Clarify the public and conceptual scheduling contract, including interface-method attribute placement and policy overrides.

Rationale

Pairwise compatibility expresses the scheduling invariant directly and remains correct when active requests have different interleaving policies. Aggregate counters avoid scanning the common single-request and read-only batch paths.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR fixes a scheduling correctness issue in the Orleans runtime where writable requests could be admitted while a read-only request was still active (when multiple [ReadOnly] requests were running and the “representative” blocker completed first). It updates the activation admission logic to evaluate compatibility against all active requests while preserving fast paths for common cases, and adds targeted tests and documentation clarifications around the scheduling contract.

Changes:

• Update ActivationData request admission to use active-request counters for common paths and pairwise compatibility checks against all running requests for heterogeneous interleaving scenarios.
• Add a comprehensive internal test suite (plus test grains and interfaces) covering read-only flagging semantics and admission ordering across ReadOnly, AlwaysInterleave, Reentrant, and MayInterleave.
• Clarify public and internal scheduling documentation and add remarks to ReadOnlyAttribute describing the intended contract and attribute placement.

Show a summary per file

File	Description
test/Orleans.Runtime.Internal.Tests/ReadOnlyAttributeTests.cs	Adds deterministic functional tests for read-only request flagging and admission ordering across multiple interleaving policies.
test/Grains/TestGrains/ReadOnlySchedulingGrain.cs	Introduces test grains used to block/unblock operations and exercise scheduling/interleaving behavior.
test/Grains/TestGrainInterfaces/IReadOnlySchedulingGrain.cs	Adds test grain interfaces with attribute combinations to validate effective request options and scheduling rules.
src/Orleans.Runtime/Catalog/ActivationData.cs	Fixes request admission logic to preserve read-only isolation and handle heterogeneous interleaving cohorts correctly.
src/Orleans.Core.Abstractions/Concurrency/GrainAttributeConcurrency.cs	Adds remarks clarifying the ReadOnlyAttribute scheduling contract and intended usage.
docs/site/src/content/docs/implementation/scheduler.md	Updates internal scheduler documentation to reflect pairwise compatibility evaluation.
docs/site/src/content/docs/grains/request-scheduling.md	Clarifies user-facing ReadOnly scheduling semantics and how it interacts with other interleaving policies.

Review details

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[github-actions]

Code coverage

77.65% line coverage - 97,649 / 125,759 lines

Coverage details

• Source files: 1,660
• Scope: every CI test matrix job, including providers, CodeGen, .NET 8/10, Linux, Windows, and macOS
• Sources: maintained repository code under src/ loaded by the complete CI test scope
• Commit: 9c6d7b63d6
• View the test run and merged coverage artifact
• View the coverage reporting run

[Copilot]

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Lite

[ReubenBond]

Documentation run 32833379243 is failing deterministically because six NuGet-link allowlist entries now return 200 and are reported as stale. Current main includes #10818, which removes those recovered allowlist entries. I am rebasing this branch onto current main and will push the refreshed branch after the solution build succeeds.

[Copilot]

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0 new
• Review effort level: Lite