refactor(runtime): include file grain storage

[ReubenBond]

Summary

• compile the file grain storage provider into Microsoft.Orleans.Runtime
• remove the standalone Microsoft.Orleans.Persistence.FileStorage project and package documentation
• relocate the provider tests alongside the core test projects and publish the API through the Runtime surface

Rationale

File grain storage is a local, single-silo development and testing capability. Shipping it with the runtime makes it available through the existing server dependency and avoids introducing a separate package for the provider added in #9537.

Microsoft Reviewers: Open in CodeFlow

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Pull request overview

This PR refactors the file-based grain storage provider by moving it into Microsoft.Orleans.Runtime, removing the standalone Microsoft.Orleans.Persistence.FileStorage project/package, relocating tests into the core test set, and exposing the provider via the runtime API surface.

Changes:

• Added Orleans.Persistence.FileStorage implementation and registration extensions directly to src/Orleans.Runtime.
• Moved/added conformance + boundary + registration tests under test/Orleans.Persistence.FileStorage.Tests and updated the solution to include them.
• Updated runtime/package documentation and API surface, and removed docs entries related to the standalone unpublished package.

Show a summary per file

File	Description
test/Orleans.Persistence.FileStorage.Tests/RecordingGrainStorageSerializer.cs	Adds a test serializer used to validate binary payload round-tripping behavior.
test/Orleans.Persistence.FileStorage.Tests/Orleans.Persistence.FileStorage.Tests.csproj	Updates test dependencies to reference Orleans.Runtime now that the provider is compiled into it.
test/Orleans.Persistence.FileStorage.Tests/FileSiloBuilderExtensionsTests.cs	Adds coverage for keyed/unkeyed DI registration behavior of the new runtime-hosted provider.
test/Orleans.Persistence.FileStorage.Tests/FileGrainStorageTestFixture.cs	Adds a shared fixture and helpers for in-process storage conformance/boundary tests.
test/Orleans.Persistence.FileStorage.Tests/FileGrainStorageOptionsTests.cs	Adds validation tests for RootDirectory configuration rules.
test/Orleans.Persistence.FileStorage.Tests/FileGrainStorageConformanceTests.cs	Adds conformance tests via the persistence test kit runner.
test/Orleans.Persistence.FileStorage.Tests/FileGrainStorageBoundaryTests.cs	Adds boundary/robustness tests for unsafe identities, invalid records, lifecycle behavior, etc.
src/Orleans.Runtime/README.md	Documents how to configure file grain storage now that it ships with Microsoft.Orleans.Runtime.
src/Orleans.Runtime/Persistence/FileStorage/FileSiloBuilderExtensions.cs	Adds silo/service collection extension methods to register file grain storage.
src/Orleans.Runtime/Persistence/FileStorage/FileGrainStorageOptions.cs	Introduces options + configuration validator for the provider.
src/Orleans.Runtime/Persistence/FileStorage/FileGrainStorageFactory.cs	Adds DI factory for constructing the provider with named options/cluster options.
src/Orleans.Runtime/Persistence/FileStorage/FileGrainStorage.cs	Adds the provider implementation (record format, hashing to filenames, ETag enforcement, lifecycle directory creation).
src/File/Orleans.Persistence.FileStorage/README.md	Removes standalone package README (provider is now documented under runtime).
src/File/Orleans.Persistence.FileStorage/Orleans.Persistence.FileStorage.csproj	Removes the standalone provider project/package.
src/api/Orleans.Runtime/Orleans.Runtime.cs	Publishes the provider API surface via Orleans.Runtime public API.
Orleans.slnx	Removes the standalone provider project and relocates provider tests into the core test set.
docs/site/src/data/unpublished-api-packages.json	Removes the unpublished package entry for the now-removed standalone package.
docs/site/src/data/external-link-allowlist.json	Removes the NuGet allowlist entry for the now-removed standalone package.
docs/site/src/content/docs/resources/nuget-packages.md	Updates package listing to point users to Microsoft.Orleans.Runtime for file storage.

Review details

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• Files reviewed: 9/19 changed files
• Comments generated: 0
• Review effort level: Lite

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Code coverage

77.56% line coverage - 97,422 / 125,601 lines

Coverage details

• Source files: 1,659
• 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: 61fc2bac67
• View the test run and merged coverage artifact
• View the coverage reporting run

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Review details

• Files reviewed: 9/19 changed files
• Comments generated: 0 new
• Review effort level: Lite