DummyDeleteLayer (introduced in cd7bcdd1f5) was applied unconditionally in
build_operator, disabling all OpenDAL delete operations. Gate it behind the
`_DATABEND_INTERNAL_DISABLE_DELETE` environment variable so deletes work by
default and are only blocked when explicitly enabled.

Operators are cached globally by StorageParams in the low-level storage crate,
which has no session context, so a process-level env var (matching the existing
`_DATABEND_INTERNAL_*` toggles in the same function) is the correct granularity
rather than a per-session SQL setting. Truthy values: 1/true/on/yes
(case-insensitive).

Rewrite the integration test to exercise the real init_operator path against
S3/MinIO: verify deletes succeed by default and are rejected with Unsupported
when the env var is set. Tests skip gracefully when no S3 endpoint is reachable.