fix: carry request.cf through debug port capnp layer

[penalosa]

Summary

• Fixes request.cf being lost when requests go through the debug port (used by Miniflare's native dev registry for cross-worker service bindings)
• The cf blob was already being passed to startRequest(metadata) by Fetcher::getClientWithTracing() but WorkerdBootstrapSubrequestChannel ignored it entirely

Changes

Sender side (workerd-debug-port-client.c++):

• CfInjectingWorkerInterface: thin WorkerInterface wrapper that injects MF-CF-Blob header into outgoing HTTP requests when metadata.cfBlobJson is present
• WorkerdBootstrapSubrequestChannel::startRequest() now wraps the RpcWorkerInterface with CfInjectingWorkerInterface when cf blob is provided

Receiver side (server.c++):

• CfAwareHttpService: wraps the target service's HTTP interface, extracts MF-CF-Blob header from incoming requests, and passes cf to SubrequestMetadata
• EventDispatcherImpl defers WorkerInterface creation until the HTTP request arrives (stores SubrequestChannel instead of WorkerInterface), enabling cf blob extraction
• startEvent() passes kj::addRef(*service) instead of service->startRequest({})

Context

Part of cloudflare/workers-sdk#12600 — refactoring the Miniflare dev registry to use native debug port RPC instead of a Node.js HTTP proxy.

[codspeed-hq]

Merging this PR will degrade performance by 21.01%

❌ 1 regressed benchmark
✅ 69 untouched benchmarks
⏩ 129 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	bodyWithHeaders[Response]	30.3 µs	38.3 µs	-21.01%

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_{Comparing penalosa/debug-port-cf-blob (3176119) with main (acdca96)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩