call ds.trackReqHeaders() as needed in app middleware

[siosonel]

Description

Call the optional ds.trackReqHeaders as needed in app.middleware, so that datasets like GDC can add request header fields that are required by their API.

Addresses https://gdc-ctds.atlassian.net/browse/FEAT-888 and https://gdc-ctds.atlassian.net/browse/FEAT-889.

Checklist

Check each task that has been performed or verified to be not applicable.

• Tests: Added and/or passed unit and integration tests, or N/A
• Todos: Commented or documented, or N/A
• Notable Changes: updated release.txt, prefixed a commit message with "fix:" or "feat:", added to an internal tracking document, or N/A
• Rust: Checked to see whether Rust needs to be re-compiled because of this PR, or N/A

[Copilot]

Pull request overview

This PR adds support for datasets (notably GDC) to optionally track incoming request headers in app middleware so downstream external-API calls can include required header fields.

Changes:

• Add trackReqHeaders(req,res) and update getHostHeaders typing/docs on the dataset type.
• Invoke dataset header tracking alongside abort-controller setup in server/src/app.middlewares.js.
• Add an Unreleased changelog entry noting the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
shared/types/src/dataset.ts	Adds trackReqHeaders to the dataset type and updates docs/typing for external API host/headers helpers.
server/src/app.middlewares.js	Hooks optional dataset request-header tracking into middleware and refactors abort-controller helper.
CHANGELOG.md	Adds an Unreleased note about calling ds.trackReqHeaders() in middleware.

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