Fix double URL-encoding of paths with spaces in category-p2-metadata

[renemuehle]

Problem

tycho-p2-plugin:category-p2-metadata fails when the build/output path contains a space
(or any character requiring percent-encoding), e.g. a Jenkins workspace named My Build:

ProvisionException: Error reading update site file:/.../my%2520site/category.xml
-> "P2 publisher return code was 1"

Root cause

CategoryP2MetadataMojo#addArguments emits the -metadataRepository / -categoryDefinition
arguments via toURI().toURL().toExternalForm() — an encoded URI string (space → %20).
CategoryPublisherApplication / AbstractPublisherApplication then parse them with
org.eclipse.core.runtime.URIUtil.fromString(String), whose javadoc states it is for
unencoded strings and:

This method must not be called with a string that already contains an encoded URI,
since this will result in the URI escape character ('%') being escaped itself.

So %20 is re-encoded to %2520, and the publisher looks for a non-existent directory
literally named my%20site.

Fix

Use URIUtil.toUnencodedString(URI) — the documented counterpart of URIUtil.fromString —
so producer and consumer use the matching encoding convention. Verified that the same
fromString logic then yields correct single-encoding on both Windows and Linux.

Reproduction (independent of any wrapper plugin)

1. Take a p2 metadata repo (content.jar + artifacts.jar) and a category.xml, placed under
   a directory whose absolute path contains a space, e.g. .../my site/repository/.
2. Run tycho-p2-plugin:category-p2-metadata with target = that repo dir and
   categoryDefinition = that category.xml.
3. It fails as above; the same inputs under a space-free path succeed.

Affected

Reproduced with 5.0.3; the offending code is unchanged on main.

Related: #5939, #652 (same family of path-encoding issues, different code path).

[laeubi]

@renemuehle can you provide an integration-test to demonstrate the issue and make sure it does not brake in the future?