Fix(Fabric): Skip Catalog Jump

[fresioAS]

Description

Reduces excessive catalog-switching churn in the Fabric engine adapter.

Root cause:
get_current_catalog() was inherited from GetCurrentCatalogFromFunctionMixin, executing SELECT db_name() on every call — including the two calls made by the set_catalog decorator (switch-in + restore) wrapping every catalog-scoped operation. For Fabric, each SQL statement runs in an independent session, so this also triggered a connection close() + reopen on every single restore, even when restoring to the same catalog.

Changes:

1. Override get_current_catalog() to read thread-local state (_target_catalog) instead of querying the database. Eliminates all SELECT db_name() round-trips.

2. Treat the default catalog as neutral (None) via _normalize_catalog(). The configured database is the implicit connection default — there is no meaningful distinction between "connected to default" and "no catalog set", so both map to None. This prevents the decorator from seeing a mismatch and unnecessarily switching back to the default after every operation.

3. Lazy connection management via _connected_catalog tracking. set_current_catalog() now distinguishes between the logical catalog target (_target_catalog) and the physical catalog the open connection is using (_connected_catalog). The connection is only closed and reopened when the physical connection actually needs to change:

   • Restoring to neutral after a catalog-scoped call: _target_catalog is updated, but the open connection is kept alive for reuse on the next call to the same catalog.
   • Switching to the same non-default catalog a second time: no close, the existing connection is reused directly.
   • Switching to a different catalog: one close + reopen as before.

4. Fix _drop_catalog to check both _connected_catalog and get_current_catalog() (with default-db fallback) before closing all connections, so dropping the default warehouse is still handled correctly.

Test Plan

• Unit tests added covering: neutral state behaviour, no DB queries from get_current_catalog, default catalog clearing, restore-to-neutral without connection close, same-catalog reuse (1 close for N consecutive ops), and cross-catalog switching (1 close per genuine switch).
• Tested and verified on live data locally. Catalog-switching log lines reduced from 175 → 51 per plan run.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO