Replace the tests that read the removed table-config endpoint

[robinskil]

Problem

Five integration tests read GET /api/admin/table-config. The server no longer supports this endpoint.

The server answers with this message:

Table configuration is no longer supported. A table's definition is engine bookkeeping rather than an API contract.

Commit 29bc0d0 (#333) removed the endpoint. See beacon-server/beacon-server/src/axum/admin/tables.rs:42.

Each test still parses the old body. Each test fails.

Failed tests

• test_etl_pipeline.py::test_full_etl_pipeline
• test_lance_tables.py::test_plain_create_table_is_lance_backed
• test_lance_tables.py::test_ctas_creates_lance_table
• test_mysql.py::test_mysql_credentials_redacted
• test_sql_databases.py::test_postgres_credentials_redacted_in_config

Task

1. Replace each engine assertion with SHOW EXTENSIONS FOR <table>.
2. Replace each schema read with GET /api/table-schema.
3. Find another source for the redacted secret value.
4. Run the suite again.

Environment

Branch features/reimplement-atlas. Image beacon-integration:local.