fix(coding-agent): disable undici body/headers idle timeouts on global dispatcher

Long local-LLM SSE streams (e.g. vLLM buffering large tool calls) exceeded
undici's default 300s bodyTimeout and aborted with UND_ERR_BODY_TIMEOUT.
retry.provider.timeoutMs cannot lift this cap because it controls the
provider SDK's AbortController deadline, not undici's per-socket idle timer.

Pass { bodyTimeout: 0, headersTimeout: 0 } to EnvHttpProxyAgent. Provider
SDKs still enforce their own timeouts via retry.provider.timeoutMs.

closes #3715

packages/coding-agent/src/cli.ts

@@ -13,6 +13,10 @@ process.title = APP_NAME;
 process.env.PI_CODING_AGENT = "true";
 process.emitWarning = (() => {}) as typeof process.emitWarning;
 
-setGlobalDispatcher(new EnvHttpProxyAgent());
+// bodyTimeout/headersTimeout default to 300s in undici; long local-LLM stalls
+// (e.g. vLLM buffering a large tool call) exceed that and abort the SSE stream
+// with UND_ERR_BODY_TIMEOUT. Disable both — provider SDKs enforce their own
+// AbortController-based deadlines via retry.provider.timeoutMs.
+setGlobalDispatcher(new EnvHttpProxyAgent({ bodyTimeout: 0, headersTimeout: 0 }));
 
 main(process.argv.slice(2));