[internal] Move dev server helper

[yoannmoinet]

Stacked on

• [internal] Change how we initialise plugins #151

What and why?

In preparation of [feat] Add synthetics plugin we extract the dev server implementation into a core helper.

How?

It's mostly a code move with some small additions to allow for a more basic utilisation.

• streamlining server responses.
• adding a middleware feature so yarn cli dev-server can implement its templating feature.
• adding a routes feature so @dd/synthetics-plugin can implement its routes for inter communication with datadog-ci.

TODO

• Add tests

[Copilot]

Pull Request Overview

This PR extracts the dev server functionality into a shared helper module to streamline server responses and support middleware and routing for the upcoming synthetics plugin integration.

• Moved file serving logic from the dev-server command to a new server helper.
• Introduced middleware support for templating and logging modifications.
• Added basic routing in the server helper for future inter-component communication.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/tools/src/commands/dev-server/index.ts	Updated to use the new runServer helper with middleware support replacing inlined file serving.
packages/core/src/helpers/server.ts	New helper module centralizing file preparation, server creation, and route handling.

Comments suppressed due to low confidence (2)

packages/tools/src/commands/dev-server/index.ts:110

• [nitpick] Consider renaming the variable 'c' to 'statusColor' for improved clarity.

const c = { 200: chalk.green, 404: chalk.yellow.bold, 500: chalk.red.bold }[statusCode] || chalk.white;

packages/core/src/helpers/server.ts:57

• The staticPath is resolved using process.cwd() here, whereas the original dev-server logic used the ROOT constant. This discrepancy might lead to inconsistent behavior if process.cwd() differs from ROOT.

const staticPath = root ? path.isAbsolute(root) ? root : path.resolve(process.cwd(), root) : process.cwd();