feat(streaming): add grain-backed stream checkpointer

[ReubenBond]

Split from #8967.

Adds a provider-independent stream queue checkpointer backed by Orleans grain storage and configured through ISiloPersistentStreamConfigurator, allowing any persistent stream provider to use it.

The implementation supports per-stream-provider grain storage selection, validates that the selected keyed storage provider is registered, and uses collision-safe identities across services, stream providers, partitions, and storage providers. Checkpoint updates use compare-and-swap semantics to prevent stale writers from regressing persisted offsets.

Event Hubs can opt into the grain-backed checkpointer with numeric checkpoint ordering while retaining its existing Azure Table Storage checkpointer as the default. This also unblocks the Kinesis provider in #8967.

Follow-up: #10355 generalizes the Azure Table checkpointer and is based on this PR's head commit.

[Copilot]

Pull request overview

This PR introduces a provider-independent, grain-storage-backed stream queue checkpointer to Orleans.Streaming, adds a reusable checkpoint comparer (numeric ordering for arbitrary-sized offsets), and wires the new checkpointer into Event Hubs as an opt-in alternative to the existing Azure Table checkpointer. It also adds deterministic contract tests to validate persistence, throttling, flushing, cancellation, failure propagation, and monotonic update behaviors.

Changes:

• Add GrainStreamQueueCheckpointer (+ factory/options) and supporting grains/state for persisting checkpoints via Orleans grain storage.
• Add StreamCheckpointComparers.Numeric and tests to ensure safe numeric ordering for Event Hubs-style offsets.
• Add configuration extensions + docs/tests so Event Hubs providers can opt into the grain-based checkpointer while preserving existing defaults.

Show a summary per file

File	Description
test/Orleans.Streaming.Tests/Checkpointers/StreamQueueCheckpointerTests.cs	Adds deterministic contract tests for IStreamQueueCheckpointer<string> implementations.
test/Orleans.Streaming.Tests/Checkpointers/StreamCheckpointComparersTests.cs	Adds unit tests for the new numeric checkpoint comparer behavior.
test/Orleans.Streaming.Tests/Checkpointers/GrainStreamQueueCheckpointerTests.cs	Adds tests for grain-backed checkpointer behavior and grain/state integration.
test/Extensions/Orleans.Streaming.EventHubs.Tests/CheckpointerTests/EventHubCheckpointerTests.cs	Updates EventHub checkpointer tests and adds ordering/no-advance coverage.
test/Extensions/Orleans.Streaming.EventHubs.Tests/CheckpointerTests/EventHubCheckpointerConfigurationTests.cs	Adds DI/hosting tests for selecting grain vs Azure Table checkpointers.
src/Orleans.Streaming/QueueBalancer/PersistentStreamConfiguratorExtension.cs	Adds UseGrainCheckpointer extension on ISiloPersistentStreamConfigurator.
src/Orleans.Streaming/Checkpointers/StreamCheckpointerGrainState.cs	Introduces persisted grain state model for checkpoints.
src/Orleans.Streaming/Checkpointers/StreamCheckpointerGrain.cs	Adds grain implementations to read/write checkpoints via grain storage.
src/Orleans.Streaming/Checkpointers/StreamCheckpointComparers.cs	Introduces common checkpoint comparer(s), including numeric ordering.
src/Orleans.Streaming/Checkpointers/IStreamCheckpointerGrain.cs	Introduces grain interface for checkpoint persistence (and configured variant).
src/Orleans.Streaming/Checkpointers/GrainStreamQueueCheckpointerOptions.cs	Adds options for persistence interval, storage provider selection, and ordering.
src/Orleans.Streaming/Checkpointers/GrainStreamQueueCheckpointerFactory.cs	Adds factory for creating grain-backed checkpointers per partition.
src/Orleans.Streaming/Checkpointers/GrainStreamQueueCheckpointer.cs	Implements grain-backed checkpointing with throttling/flush semantics.
src/Azure/Orleans.Streaming.EventHubs/README.md	Documents opting into grain storage for Event Hubs checkpoints.
src/Azure/Orleans.Streaming.EventHubs/Providers/Streams/EventHub/EventHubStreamBuilder.cs	Adds UseGrainCheckpointer for Event Hubs stream configurator (defaults numeric ordering).
src/api/Orleans.Streaming/Orleans.Streaming.cs	Updates public API surface for new checkpointer/grain types/options/comparers/extensions.
src/api/Azure/Orleans.Streaming.EventHubs/Orleans.Streaming.EventHubs.cs	Updates Event Hubs public API surface to include UseGrainCheckpointer.

Copilot's findings

• Files reviewed: 17/17 changed files
• Comments generated: 5