### What changes were proposed in this pull request?
when the current catalog is session catalog, get/set the current namespace from/to the `SessionCatalog`.
### Why are the changes needed?
It's super confusing that we don't have a single source of truth for the current namespace of the session catalog. It can be in `CatalogManager` or `SessionCatalog`.
Ideally, we should always track the current catalog/namespace in `CatalogManager`. However, there are many commands that do not support v2 catalog API. They ignore the current catalog in `CatalogManager` and blindly go to `SessionCatalog`. This means, we must keep track of the current namespace of session catalog even if the current catalog is not session catalog.
Thus, we can't use `CatalogManager` to track the current namespace of session catalog because it changes when the current catalog is changed. To keep single source of truth, we should only track the current namespace of session catalog in `SessionCatalog`.
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
Newly added and updated test cases.
Closes#25903 from cloud-fan/current.
Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Gengliang Wang <gengliang.wang@databricks.com>
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