fix: Return 404 for missing tables in get_table_count API

[sentry]

This PR addresses issue DALGO-BACKEND-2H1 by changing the behavior of the /api/warehouse/table_count/{schema_name}/{table_name} endpoint when a requested table does not exist.

Problem: Previously, if a client requested the row count for a table that did not exist in the warehouse (e.g., intermediate.int_2digit), the underlying database (PostgreSQL or BigQuery) would raise an error like "relation does not exist". This error was caught by a generic exception handler in the API, resulting in an HTTP 500 Internal Server Error.

Solution:

1. Introduced a new custom exception TableNotFoundError in ddpui/utils/warehouse/exceptions.py.
2. Modified get_total_rows methods in both ddpui/utils/warehouse/old_client/postgres.py and ddpui/utils/warehouse/old_client/bigquery.py to specifically catch database-specific "table not found" errors (psycopg2.errors.UndefinedTable for Postgres and google.cloud.exceptions.NotFound for BigQuery).
3. These specific exceptions are now re-raised as the common TableNotFoundError.
4. The get_table_count endpoint in ddpui/api/warehouse_api.py was updated to catch TableNotFoundError and return an HttpError(404, "Table not found").

Impact: This change provides a more semantically correct and user-friendly response (HTTP 404 Not Found) when a requested table is unavailable, improving API robustness and client-side error handling.

Fixes DALGO-BACKEND-2H1