Add settle window to DataLoader batch dispatch

[michaelstaib]

Summary

Adds a configurable settle window to the BatchDispatcher so a batch only becomes eligible for dispatch once it has gone a minimum amount of time without receiving new items. This widens the aggregation window under concurrent load, so fewer, larger batches reach the DataLoader.

Closes #9793

[Copilot]

Pull request overview

This PR introduces a configurable settle window to BatchDispatcher so batches become dispatch-eligible only after a minimum quiet period, improving aggregation under concurrent load while preserving age-based forced dispatch semantics.

Changes:

• Added BatchSettleTimeUs plus “was set” tracking to BatchDispatcherOptions so default(BatchDispatcherOptions) yields documented defaults.
• Updated BatchDispatcher dispatch eligibility and coordinator loop to incorporate settle time and replace the prior idle-cycle heuristic with a time-based no-dispatch streak/backoff strategy.
• Added/updated tests for settle window behavior and option default handling, plus an additional experiment-style test file.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/HotChocolate/Core/src/Types/Fetching/BatchDispatcher.cs	Adds settle-window gating to dispatch eligibility and revises coordinator backoff/evaluation signaling.
src/HotChocolate/Core/src/Types/Fetching/BatchDispatcherOptions.cs	Adds BatchSettleTimeUs and explicit default handling for default(...) scenarios.
src/HotChocolate/Core/src/Types/Fetching/BatchDispatcher.IBatchScheduler.cs	Removes now-unused enqueue timestamp tracking.
src/HotChocolate/Core/src/Types/Fetching/BatchDispatcher.ExecutorSession.cs	Removes now-unused subscribe timestamp tracking.
src/HotChocolate/Core/test/Fetching.Tests/BatchDispatcherTests.cs	Adds tests for settle window inclusion of late items and default/zero option semantics.
src/HotChocolate/Core/test/Execution.Tests/Integration/DataLoader/DataLoaderBatchAggregationExperimentTests.cs	Adds measurement-only experiment tests for batch aggregation behavior under load.

Comments suppressed due to low confidence (2)

src/HotChocolate/Core/src/Types/Fetching/BatchDispatcherOptions.cs:19

• The XML doc says "class" but BatchDispatcherOptions is a struct. Updating this avoids misleading generated docs.

    /// <summary>
    /// Initializes a new instance of the <see cref="BatchDispatcherOptions"/> class.
    /// </summary>

src/HotChocolate/Core/src/Types/Fetching/BatchDispatcher.cs:165

• This comment implies we only return when in-flight dispatches are completed, but the code returns whenever openBatches == 0 (relying on dispatch completion continuations to re-signal the coordinator). Please update the comment to match the actual control flow.

            // If we have no open batches to evaluate and all in-flight dispatches
            // are completed, we can stop and wait for another signal.
            // If there are in-flight dispatches still running we also stop and
            // wait for their completion signal.
            if (openBatches == 0)

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Patch coverage

100.0% of changed lines covered (46/46)

File	Covered	Changed	Patch %
src/HotChocolate/Core/src/Types/Fetching/BatchDispatcher.cs	27	27	100.0% 🟢
…/Core/src/Types/Fetching/BatchDispatcherOptions.cs	19	19	100.0% 🟢

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Project coverage: 53.8% (231332/430063 lines)