fix(maintenance): open namespace-backed Lance tables via catalog entry

[mikewhb]

Root cause: maintenance table functions (compact / cleanup / optimize_index / set_auto_cleanup / show_auto_cleanup) bind by passing the first argument through ResolveLanceDatasetUri() and then call LanceOpenDataset(bind_data.dataset_uri) at execute time.

For namespace-backed Lance tables, DefaultCatalogResolver returns LanceTableEntry::DatasetUri(), which for REST namespaces is the "virtual" handle 'endpoint + "/" + table_id' (e.g. 'http://host:2333/cat$sch$tbl'). lance-io cannot open that handle directly -- the real cos:// / file:// location is only reachable through the namespace REST API via LanceOpenDatasetForTable().

As a result, OPTIMIZE / VACUUM LANCE / SHOW MAINTENANCE / ALTER TABLE SET AUTO_CLEANUP / ALTER TABLE UNSET AUTO_CLEANUP all fail with 'IO Error: Failed to open Lance dataset: http://...' whenever the target is a REST namespace table. Directory namespaces are unaffected because their DatasetUri() is already the real on-disk path.

Fix: at execute time, re-resolve the original first-argument literal as a Lance catalog entry and route through LanceOpenDatasetForTable(), which handles the namespace REST API path. Fall back to the old LanceOpenDataset(dataset_uri) when the input is a plain path literal so the existing 'open by path' contract is preserved.

Changes:

• Promote TryResolveLanceTableEntry() from a static helper in lance_search.cpp to a public symbol in lance_common.{hpp,cpp} so the search and maintenance code paths share one canonical implementation. The shared version uses OnEntryNotFound::RETURN_NULL and only catches ParserException, so genuine errors (auth failures, network errors, catalog corruption) are no longer silently swallowed by 'catch(...)'.
• lance_maintenance.cpp: store the original first-argument literal alongside dataset_uri in LanceMaintenanceBindData, LanceAutoCleanupSetBindData, and LanceAutoCleanupShowBindData. At execute time, try TryOpenDatasetForMaintenanceInput() first (TryResolveLanceTableEntry + LanceOpenDatasetForTable) and only fall back to LanceOpenDataset(dataset_uri) when the input does not name a Lance catalog entry.
• The displayed Target column now reflects the resolved real URI returned by LanceOpenDatasetForTable() instead of the virtual namespace handle.
• Add test/sql/namespace_rest_maintenance.test exercising compact, cleanup, set / unset / show auto_cleanup against a REST namespace table. Gated by LANCE_TEST_NAMESPACE=1, mirroring namespace_rest_attach.test.

Compatibility:

• Path-based callers (e.g. OPTIMIZE 'cos://bucket/x.lance') are unchanged: TryResolveLanceTableEntry short-circuits on inputs containing '/', '\', or '://' and the executor falls back to the original LanceOpenDataset() path.
• Directory-namespace callers (e.g. OPTIMIZE dir_ns.main.tbl) are unchanged: dir-namespace tables are not IsNamespaceBacked(), so LanceOpenDatasetForTable() forwards to LanceOpenDataset(table.DatasetUri()) -- the same real path that the previous code was already using.

[Xuanwo]

Thank you for working on this!