perf(amp): avoid fibers in future adapter

[simPod]

Context

AmpFutureAdapter created an AMPHP Fiber for every promise continuation and aggregate. Wide GraphQL result graphs therefore retained many Fiber states and event-loop callbacks even when the underlying Futures only represented batched work.

Decision

Compose and aggregate Futures through map() and catch() callbacks, settling one DeferredFuture while retaining Promise fulfillment, rejection, and nested-Future flattening semantics.

Consequences

• Reduces per-field allocation and event-loop scheduling for wide asynchronous result graphs.
• Preserves deferred completion for genuinely asynchronous resolvers.
• Adds coverage for nested Future fulfillment/recovery, fulfillment errors, aggregate key preservation, aggregate rejection, and resolution after rejection.

Benchmark

On the same wide GraphQL result graph with 838 parent objects and three Future-backed fields:

Adapter	CPU time	Peak memory
Synchronous adapter	1.48s	122MB
Existing AMPHP adapter	2.11s	274MB
This change	1.67-1.70s	149MB

Relative to the existing AMPHP adapter, this reduces CPU time by about 19% and peak memory by about 46%. The remaining overhead compared with the synchronous adapter is about 13-15% CPU and 22% memory.

Disclosure

This change was identified while migrating a production GraphQL application that is not yet fully migrated to asynchronous resolving. It is a step toward that migration, rather than a claim of end-to-end asynchronous production use.

[Copilot]

Pull request overview

This PR refactors the AMPHP v3 AmpFutureAdapter to compose and aggregate Future continuations without creating a new Fiber per continuation/aggregate, aiming to reduce allocations and event-loop overhead for wide async GraphQL result graphs.

Changes:

• Reworked AmpFutureAdapter::then() to settle a single DeferredFuture via Future::map()/catch() observation rather than async + await.
• Reimplemented AmpFutureAdapter::all() to resolve/reject by observing each input Future, preserving array keys and avoiding aggregate await.
• Added/expanded PHPUnit coverage for nested-future flattening, fulfillment callback exceptions, key preservation, aggregate rejection, and “stay rejected” behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Executor/Promise/Adapter/AmpFutureAdapter.php	Removes per-continuation fibers by observing futures with map()/catch() and settling a shared DeferredFuture.
tests/Executor/Promise/AmpFutureAdapterTest.php	Adds tests covering nested future unwrapping, rejection/exception behavior, key preservation, and aggregate failure semantics.
CHANGELOG.md	Documents the behavioral/performance fix under Unreleased.

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