[bbdu3]

Summary

Add an optional integration point for propagating execution context into SDK-managed handler fibers.

With OpenTelemetry’s default fiber-bound storage, accessing context inside an uninitialized handler fiber raises a warning. When an application converts warnings into exceptions, the request fails with -32603. This fix allows applications to configure context propagation explicitly; it does not enable OpenTelemetry integration automatically.

Fixes #583

Changed

• Add FiberFactoryInterface and Builder::setFiberFactory(), wired into Protocol and StatelessProtocol, including buildStateless().
• Invoke the factory in the parent context and require an unstarted fiber that preserves the callback’s return value, exceptions and suspensions.
• Document an OpenTelemetry implementation that captures the parent context, activates it inside the handler fiber and detaches it in finally. Default fiber-bound storage retains the context across suspension and resumption, avoiding additional lifecycle hooks at each start() or resume().
• Preserve existing behavior when no factory is configured, with no additional package dependency or protocol changes.

PR #616 addresses #615 by coordinating fiber execution with an external event loop. This PR addresses context propagation and leaves scheduling unchanged. Both touch handler-fiber creation, so combining them may require adapting the factory’s integration point; their combined implementation has not been tested.

Test

• Focused regression suite: 250 tests passed, including 16 new cases covering both protocol eras, buildStateless(), HTTP and stdio, interleaved requests, suspension/resumption, handler exceptions, and cleanup when suspended fibers are discarded.
• OpenTelemetry reproduction: an isolated local probe using open-telemetry/context 1.5.0 passed 12 before/after scenarios with warnings promoted to exceptions. The default configuration reproduced the reported failure; the configured factory preserved context and completed cleanup without those warnings.
• Static analysis and documentation: PHPStan, PHP CS Fixer checks on changed PHP files, and the strict documentation build passed.