fix(FileSink): truncate existing file when using writer()

The FileSink's flags() method was ignoring the truncate option, which
defaults to true. This caused existing files to not be truncated when
writing via file.writer(), so if the new content was shorter than the
old content, the tail of the old content would remain.

Closes #25968

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/bun.js/webcore/FileSink.zig

@@ -54,9 +54,11 @@ pub const Options = struct {
     mode: bun.Mode = 0o664,
 
     pub fn flags(this: *const Options) i32 {
-        _ = this;
-
-        return bun.O.NONBLOCK | bun.O.CLOEXEC | bun.O.CREAT | bun.O.WRONLY;
+        var f: i32 = bun.O.NONBLOCK | bun.O.CLOEXEC | bun.O.CREAT | bun.O.WRONLY;
+        if (this.truncate) {
+            f |= bun.O.TRUNC;
+        }
+        return f;
     }
 };
 

test/regression/issue/25968.test.ts

@@ -0,0 +1,50 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+test("FileSink truncates existing file", async () => {
+  using dir = tempDir("issue-25968", {});
+  const filePath = `${dir}/file.txt`;
+
+  // Write initial long content
+  const file = Bun.file(filePath);
+  await Bun.write(file, "Long content");
+
+  // Write shorter content using writer
+  const writer = file.writer();
+  writer.write("Short");
+  writer.end();
+
+  // Verify the file is truncated and only contains the new content
+  const result = await file.text();
+  expect(result).toBe("Short");
+});
+
+test("FileSink truncates when writing to existing file via spawn", async () => {
+  using dir = tempDir("issue-25968-spawn", {});
+  const filePath = `${dir}/file.txt`;
+
+  await using proc = Bun.spawn({
+    cmd: [
+      bunExe(),
+      "-e",
+      `
+const file = Bun.file("${filePath.replace(/\\/g, "\\\\")}");
+await Bun.write(file, "Long content that is longer");
+const writer = file.writer();
+writer.write("Short");
+writer.end();
+console.log(await file.text());
+`,
+    ],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout.trim()).toBe("Short");
+  expect(exitCode).toBe(0);
+});