Fix bad error reporting when registering a (faulty) RRD URI built from a relative path

[abey79]

Related

• fixes https://linear.app/rerun/issue/RR-3234/calling-registration-with-relative-path-on-oss-server-leads-to-opaque
• sibling https://github.com/rerun-io/dataplatform/pull/2226

What

Sticking "file:// in front of a relative path is a footgun, because the resulting URI is valid, but the first path element is considered a hostname (uri in the form of file://hostname/path/to/file.txt). These were previously accepted by /RegisterWithDataset and silently dropped by the OSS server.

This PR:

• hardens the URI handling in OSS server
• adds a redap test to ensure not_found errors are returned for bad URIs

[github-actions]

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Result	Commit	Link	Manifest
✅	359506d	https://rerun.io/viewer/pr/12309	+nightly +main

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[grtlr]

I've removed the consider-patch label as per our discussion.