feat: add diagnostic channels for request lifecycle

[OussemaNehdi]

Summary

Adds diagnostics_channel support for Express request lifecycle, as requested
in #6353. This enables APM and monitoring tools to observe request processing
without monkey-patching.

Channels

Channel	When	Message
express.request.start	Before routing begins	{ req, res }
express.request.finish	Response fully sent	{ req, res }
express.request.error	Connection error (ECONNRESET, etc.)	{ req, res, error }

Design decisions

• Zero overhead: Every publish() call is guarded by hasSubscribers,
  so there is no performance cost when no diagnostic tool is listening
• on-finished for completion tracking: Uses the existing on-finished
  dependency (already used by res.sendFile) to detect when the response is
  fully sent, covering all code paths including streaming and error responses
• Complements feat: add diagnostics_channel support for app initialization #7041: PR feat: add diagnostics_channel support for app initialization #7041 adds an express.initialization channel for
  app startup. This PR covers the request lifecycle, which is the primary use
  case for APM tools like OpenTelemetry and Datadog

Example usage

const dc = require('node:diagnostics_channel');

dc.subscribe('express.request.start', ({ req, res }) => {
  req._startTime = performance.now();
});

dc.subscribe('express.request.finish', ({ req, res }) => {
  const duration = performance.now() - req._startTime;
  console.log(`${req.method} ${req.url} ${res.statusCode} ${duration.toFixed(1)}ms`);
});

Test plan

- express.request.start publishes before routing with { req, res }
- express.request.start fires for every request (verified with 2 sequential requests)
- No overhead when no subscribers (request works normally)
- express.request.finish fires after response with correct statusCode
- express.request.finish fires for 500 error responses
- express.request.finish fires for 404 responses
- express.request.error does NOT fire on normal responses
- start always fires before finish (ordering verified)
- All 1257 tests pass (1252 existing + 5 new)
- ESLint clean