fix: preserve namespace drop errors (#4099)

## Summary

- preserve typed namespace errors returned by `drop_table`
- return `TableNotFound` when dropping an absent namespace table
- cover repeated drop behavior in the namespace database test

## Validation

- `cargo test --quiet --features remote -p lancedb
database::namespace::tests::test_namespace_drop_table --lib`
- `cargo clippy --quiet --features remote -p lancedb --lib --tests -- -D
warnings`

## Context

Sophon SQL implements `DROP TABLE IF EXISTS` by matching
`lancedb::Error::TableNotFound`. The namespace database previously
wrapped this error as `Runtime`, causing cleanup to fail and mask an
earlier statement error.

rust/lancedb/src/database/namespace.rs

@@ -539,9 +539,7 @@ impl Database for LanceNamespaceDatabase {
         self.namespace
             .drop_table(drop_request)
             .await
-            .map_err(|e| Error::Runtime {
-                message: format!("Failed to drop table: {}", e),
-            })?;
+            .map_err(|e| map_namespace_lance_error(e, name))?;
 
         Ok(())
     }
@@ -1495,6 +1493,15 @@ mod tests {
             .expect("Failed to list tables");
         assert!(!table_names_after.contains(&"drop_test".to_string()));
 
+        let error = conn
+            .drop_table("drop_test", &["test_ns".into()])
+            .await
+            .expect_err("dropping a missing table should fail");
+        assert!(
+            matches!(error, Error::TableNotFound { ref name, .. } if name == "drop_test"),
+            "expected TableNotFound, got: {error:?}"
+        );
+
         // Verify: Cannot open dropped table
         let open_result = conn.open_table("drop_test").execute().await;
         assert!(open_result.is_err());