perf(pi-natives): optimized pipe I/O to use async instead of blocking workers

- Replaced blocking Tokio worker with non-blocking async I/O for pipe reading to prevent stalling the runtime when child processes keep pipes open.
- Refactored background job termination to use process group and process tree APIs instead of deprecated signal parsing, improving reliability across Unix and Windows.
- Added non-blocking pipe registration and fcntl-based flag management for Unix platforms to enable efficient async I/O without blocking workers.
- Added test case verifying read_output cancellation before pipe EOF to ensure proper cleanup behavior.

packages/natives/test/native.test.ts

@@ -3,6 +3,7 @@ import * as fs from "node:fs/promises";
 import * as os from "node:os";
 import * as path from "node:path";
 import {
+	executeShell,
 	FileType,
 	fuzzyFind,
 	type GlobMatch,
@@ -385,6 +386,29 @@ describe("pi-natives", () => {
 			}
 		});
 	});
+
+	describe("shell", () => {
+		it("should time out background workloads without leaving delayed writers behind", async () => {
+			if (process.platform === "win32") {
+				return;
+			}
+
+			const markerPath = path.join(testDir, "shell-timeout-marker.txt");
+			const markerEscaped = markerPath.replace(/'/g, "'\\''");
+			await fs.rm(markerPath, { force: true });
+
+			const result = await executeShell({
+				command: `{ sleep 2; echo done > '${markerEscaped}'; } & sleep 10`,
+				cwd: testDir,
+				timeoutMs: 100,
+			});
+
+			expect(result.timedOut).toBe(true);
+
+			await Bun.sleep(3000);
+			expect(await Bun.file(markerPath).exists()).toBe(false);
+		});
+	});
 	describe("htmlToMarkdown", () => {
 		it("should convert basic HTML to markdown", async () => {
 			const html = "<h1>Hello World</h1><p>This is a paragraph.</p>";