* fix(sandbox): close AioSandbox HTTP client during provider teardown (#2872)
AioSandbox allocates a host-side agent_sandbox client (wrapping an
httpx.Client) in __init__, but AioSandboxProvider.release/destroy/shutdown
only popped provider state and tore down the backend container — the
client/transport owned by each cached AioSandbox was never explicitly
closed, accumulating unreclaimed sockets in long-running services.
- Add AioSandbox.close(): best-effort, idempotent close of the wrapped
httpx_client (falls back to top-level client.close()); errors are
logged but never raised so backend cleanup is never blocked.
- AioSandboxProvider.release()/destroy() now close the cached AioSandbox
before dropping it; shutdown() inherits this via destroy().
* fix(sandbox): close the real httpx.Client owned by AioSandbox (#2872)
The previous close() only walked one level (wrapper.httpx_client), which resolves to the Fern-generated HttpClient wrapper that has no close(). The real socket-owning httpx.Client lives one level deeper at _client_wrapper.httpx_client.httpx_client, so the close path never fired and host-side sockets still leaked.
Resolve the real httpx.Client with graceful degradation; clear self._client under the lock for use-after-close and concurrent double-close safety; mark provider release()/destroy() try/except as defense-in-depth; rewrite TestClose against the real nested structure to lock down the original no-op bug.
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