fix: ensure File transport flushes all data before emitting finish

[mldangelo]

Fixes #1504 and relates to promptfoo/promptfoo#6511

When you call logger.end() and then process.exit(), log messages get lost. This has been an open issue since 2018.

The problem is that the File transport's finish event fires when the internal PassThrough drains, not when the actual file write completes. So you think you're safe to exit, but the OS hasn't finished writing yet.

The fix adds a _final() method (the standard Node.js stream hook for this) that waits for the fs.WriteStream to finish before letting the transport emit finish.

---

To verify the fix:

const fs = require('fs');
const winston = require('winston');

const transport = new winston.transports.File({ filename: 'test.log' });
const logger = winston.createLogger({ transports: [transport] });

(async () => {
  for (let i = 0; i < 1000; i++) logger.info(`message ${i}`);
  logger.info('THE LAST MESSAGE');

  await new Promise(resolve => {
    transport.on('finish', resolve);
    logger.end();
  });

  const lines = fs.readFileSync('test.log', 'utf8').split('\n').filter(Boolean);
  console.log(`Lines written: ${lines.length}`);  // should be 1001
})();

Before: ~1 line written. After: all 1001 lines written.

Also added regression tests under "Flushing on end (issue #1504)" in file.test.js.

[mldangelo]

@DABH would you mind taking a look when you get a chance? This is a long-standing issue that's been open since 2018.

[DABH]

Happy to trust code from a fellow ICME person ;) The argument seems reasonable here and I appreciate the MWE and test. I will look to ship this out as a new minor release. Thank you for your contribution to Winston!