fix(csv-stringify): preserve header cast context when eof is false

[ujjwalnain]

With header: true and eof: false, string cast callbacks receive context.header === false for header cells. A formatter that handles headings separately therefore changes behavior when the final newline is disabled:

import { stringify } from "csv-stringify/sync";

stringify([{ name: "alice" }], {
  header: true,
  eof: false,
  cast: {
    string: (value, context) =>
      context.header ? value.toUpperCase() : value,
  },
});
// Before: "name\nalice"
// After:  "NAME\nalice"

Pass the header flag in both branches of headers(). Parameterize the existing header-context test over both EOF settings and compare exact output, including the trailing delimiter.

Validation on Node 24.19.0:

• Regression fails on the original code and passes with the fix.
• npm test --workspace=csv-stringify: TypeScript check passes; 210 passing, 1 existing pending test.
• ESLint passes for both changed files.
• Additional sync/callback checks pass for header-only and populated input with both EOF settings.