fix(cli): handle BrokenPipe gracefully in bun completions (#26097)

## Summary
- Fixes `bun completions` crashing with `BrokenPipe` error when piped to
commands that close stdout early (e.g., `bun completions | true`)
- The fix catches `error.BrokenPipe` and exits cleanly with status 0
instead of propagating the error

## Test plan
- [x] Added regression test that pipes `bun completions` to `true` and
verifies no BrokenPipe error occurs
- [x] Verified test fails with system Bun and passes with fixed build

Fixes #2977

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

test/regression/issue/02977.test.ts

@@ -0,0 +1,22 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, isPosix } from "harness";
+
+// https://github.com/oven-sh/bun/issues/2977
+test.if(isPosix)("bun completions handles BrokenPipe gracefully", async () => {
+  // Simulate piping to a command that closes stdin immediately (like `true`)
+  // This tests that bun completions doesn't crash with BrokenPipe error
+  await using proc = Bun.spawn({
+    cmd: ["sh", "-c", `SHELL=/bin/bash ${bunExe()} completions | true`],
+    env: bunEnv,
+    stderr: "pipe",
+    stdout: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // Should exit cleanly (0) instead of crashing with BrokenPipe error
+  // The stderr should NOT contain "BrokenPipe" error
+  expect(stderr).not.toContain("BrokenPipe");
+  expect(stderr).not.toContain("error");
+  expect(exitCode).toBe(0);
+});