feat: add diagnostics_channel support for app initialization

[DemianParkhomenko]

Publishes express.initialization channel when an Express app is created,
allowing instrumentation libraries to attach middleware and hooks before
any request arrives.

Route-level tracing (start/end/error per request with matched route pattern)
will be implemented in the router module using TracingChannel API.

Refs: #6353 https://github.com/pillarjs/router/pull/96/changes

[Copilot]

Pull request overview

This PR adds support for Node.js diagnostics_channel to Express, specifically implementing an express.initialization channel that publishes when an Express application is created. This allows instrumentation libraries (like OpenTelemetry) to hook into the app lifecycle before any requests are handled, enabling better observability and monitoring capabilities.

Changes:

• Added a new lib/diagnostics.js module that creates and exports the express.initialization diagnostics channel
• Modified lib/express.js to publish to the initialization channel after app.init() is called
• Added comprehensive tests for the diagnostics channel functionality

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
lib/diagnostics.js	New module that creates the express.initialization diagnostics channel
lib/express.js	Imports diagnostics module and publishes initialization event when app is created
test/diagnostics-channel.js	Test suite verifying channel publishes correctly and handles no-subscriber case

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