[pvfs] Set working directory to the catalog root in PaimonVirtualFileSystem

[thswlsqls]

Purpose

fix #9506

• Behavior change, up front: a single-segment relative path such as x.csv succeeds today and throws IOException after this change. That success writes into a table the user never named, and which table is non-deterministic — the Hadoop FileSystem cache key is scheme+authority+ugi.
• initialize() took workingDirectory from the raw URI Hadoop passes in while normalizing this.uri to scheme://authority/. Reorder so it derives from the normalized catalog root. The self-contradiction dates to [Feature] Implement Paimon Virtual Filesystem #5916.
• Reached by the documented usage in docs/docs/concepts/rest/pvfs.md lines 61-62.

Tests

• Added VirtualFileSystemTest#testWorkingDirectoryIsCatalogRoot, inherited by both subclasses: the working directory equals the catalog root, and a relative-path create lands in the table it names.
• mvn -pl paimon-vfs/paimon-vfs-hadoop clean install — 29 tests pass. No existing test initializes with a path component (MockRestVirtualFileSystemTest L119, MockRestNoCacheVirtualFileSystemTest L47), so the 27 pre-existing tests stay green.