ext_proc: onDestroy() prematurely closes gRPC stream on downstream half-close when response_body_mode is enabled

**Description**
When an HTTP/1.1 client sends a request and half-closes its request direction (SHUT_WR), Envoy's ext_proc filter destroys the filter instance (onDestroy()), which unconditionally invokes closeStreamMaybeGraceful().
Even when response_body_mode (e.g., STREAMED or BUFFERED) is configured and response-side processing is still required, Envoy terminates the gRPC stream to the external processor immediately upon the downstream request completing. As a result, the external processor never receives response-side ProcessingRequest messages.

**Repro steps**
Configure ext_proc with response_body_mode: STREAMED.
Send an HTTP request via a client that half-closes (shutdown(SHUT_WR)) after sending headers/body, then waits for the response.
Observe that the external processor receives request_headers (with end_of_stream: true) and immediately gets a gRPC stream half-close (ok == false), preventing any response-side processing.

**Expected Behavior**
A downstream client half-close on the request path should not cause Envoy to terminate the ext_proc gRPC stream if response-side external processing (response_body_mode != SKIP) is active and the upstream response has not yet been processed. The stream lifecycle should align with the full HTTP exchange, not just the decoding direction.

Start by tracing the ext_proc filter's onDestroy() and closeStreamMaybeGraceful() behavior for downstream request half-closes, using the response_body_mode handling described in the issue. Reproduce the HTTP/1.1 SHUT_WR sequence with STREAMED response processing and verify that response-side ProcessingRequest messages still arrive before the gRPC stream closes.

Written by the indexing model from the issue text.