`namespace_exists()` / `table_exists()` throw `RuntimeError` instead of `NamespaceNotFoundError` / `TableNotFoundError`

[jorritsandbrink]

As per their docstring, DirectoryNamespace.namespace_exists() and DirectoryNamespace.table_exists() are supposed to raise NamespaceNotFoundError / TableNotFoundError when the namespace/table does not exist, however, they return a RuntimeError with message "Failed to downcast NamespaceError source, falling back to runtime error. This may indicate a version mismatch.".

Repro:

import tempfile

from lance.namespace import DirectoryNamespace, NamespaceExistsRequest, TableExistsRequest


ns = DirectoryNamespace(root=tempfile.mkdtemp())
ns.namespace_exists(NamespaceExistsRequest(id=["nonexistent_namespace"]))
# ns.table_exists(TableExistsRequest(id=["nonexistent_table"]))

Result:

[2026-03-20T15:20:33Z WARN  lance::error] Failed to downcast NamespaceError source, falling back to runtime error. This may indicate a version mismatch. Source type: "Namespace 'nonexistent_namespace' not found"
Traceback (most recent call last):
  File "/home/j/repos/dlt-3546/tmp.py", line 7, in <module>
    ns.namespace_exists(NamespaceExistsRequest(id=["nonexistent_namespace"]))
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/j/.cache/uv/archive-v0/HiBtUgMOpG-E8F-FVIxL_/lib/python3.13/site-packages/lance/namespace.py", line 351, in namespace_exists
    self._inner.namespace_exists(request.model_dump())
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Namespace error: Namespace 'nonexistent_namespace' not found, /home/runner/work/lance/lance/rust/lance-namespace-impls/src/dir/manifest.rs:1722:17

Using pylance==3.0.1.