refactor: simplify / ungeneralize needs.json export of project_url

[AlexanderLanin]

Why this matters

The Needs JSON exporter currently exposes a generic helper whose field list, validation policy, and output values are passed separately. That makes a one-field customization harder to understand and makes it awkward for specialized exports that must write different metadata. It's simply super difficult to extend a generic exporter. And we only export one value.

What changed

• Replace the generic exporter helper with explicit project URL registration.
• Preserve the existing host behavior: project_url is validated and exported from Sphinx configuration.
• Allow callers to provide an explicit JSON value without mutating the active Sphinx configuration.
• Resolve the host configuration at finalization time and make repeated registration idempotent.
• Keep export values tied to the Sphinx configuration owning each Needs list.
• Add regression coverage for configured values, missing configuration, late configuration changes, and repeated registration.

[Copilot]

🟡 Changes recommended

Configuration snapshots and stacked exporter wrappers can produce stale or incorrect project URLs.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Refactors Needs JSON export registration around the SCORE-specific project_url field.

Changes:

• Adds explicit project URL export and override support.
• Adds regression tests for configuration and overrides.

File summaries

File	Description
external_needs.py	Implements project URL export registration.
test_external_needs.py	Tests missing, configured, and overridden values.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Balanced

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[github-actions]

Documentation preview for this pull request is available at:
pr-797: https://eclipse-score.github.io/docs-as-code/pr-797/