v0.41.0

Client: load a whole catalog with catalog_contents

VGI protocol stays at 2.1.0 — compatible with the current vgi DuckDB extension and 0.40.x workers.

• Client.load_catalog(attach=, previous=, transaction_opaque_data=) returns a CatalogSnapshot (schemas, version, etag, source, not_modified, fallback_reason). It makes one catalog_contents call when the attach advertises supports_catalog_contents, and uses catalog_schemas plus the per-schema RPCs otherwise, skipping kinds whose estimated_object_count is 0. catalog_contents is never sent to a worker that doesn't advertise it.
• Fallback: any failure or malformed answer falls back to the per-schema RPCs and records fallback_reason. An older snapshot is retried once.
• Revalidation: pass previous= to send its etag as if_none_match. A not_modified answer keeps the held contents.
• Transactions: a load inside a transaction always uses the per-schema RPCs.
• Fix: a malformed catalog_contents item now raises CatalogClientError instead of escaping as a bare ArrowInvalid/KeyError.
• New vgi-client catalog contents CLI command; the attach output shows supports_catalog_contents.
• The six contents_* fixture catalogs are now the cross-SDK catalog_contents contract: the vgi integration tests run them against every SDK.