fix(sdk): release completed embedded request signals

[opencode-agent]

Problem

In the embedded SDK, a live host keeps completed Web Request objects after response consumption and GC. With 200 sequential server.info() calls, 201 Request objects remain after a 100 ms idle turn; a plain handler does not show the same growth. The combination of the Effect Web handler and AbortSignal.any([source.signal, shutdown.signal]) retains requests until the host shuts down. This is distinct from the projection worker backlog in anomalyco/anomaly#623.

Fix

Bridge caller cancellation and host shutdown through a per-request controller, then detach both upstream abort listeners when the response completes or is cancelled. This keeps active-request cancellation and shutdown behavior without keeping completed requests attached to the long-lived host signal.

Verification

• New real embedded Effect SDK regression: 201 additional live Request objects on the base branch, fewer than 20 after this change; repeat runs pass.
• New active caller-abort and host-shutdown test passes.
• SDK bun typecheck, scoped oxlint, Prettier, and git diff --check pass.
• Existing transport.test.ts and promise.test.ts have three unrelated assertion failures in this checkout, reproduced on the unchanged base; this change does not add failures in those files.

This is a source fix for review, not a production deployment. The production profile's full RSS/array-buffer footprint has not been attributed solely to these request objects. Related: anomalyco/anomaly#260 and draft #51012 (its earlier approach did not fix the embedded path).

Requested by: @kitlangton (Kit via Slack)

[kitlangton]

Superseded by #53613.