fix(BA-6915): drop the column default before retyping sessions.result

The downgrade recreates the plural sessionresults type and then retypes
sessions.result into it. PostgreSQL cannot cast an existing column default
across two unrelated enum types, so the ALTER aborts with
DatatypeMismatchError before touching a row. The migration's try/except
tolerates a list of expected divergence SQLSTATEs, and 42804 is not on it,
so the whole downgrade chain stops here.

Drop the default, retype, then restore it -- the order dec0deba5893 already
uses for the same conversion. The upgrade path runs the mirror-image ALTER
with the same omission, so it gets the same treatment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/manager/models/alembic/versions/0b1efbb2db84_fix_sessionresult_enum_type_coexistence.py

@@ -41,6 +41,23 @@ def _is_expected_divergence(exc: BaseException) -> bool:
     return _sqlstate_of(exc) in _EXPECTED_DIVERGENCE_SQLSTATES
 
 
+def _retype_result_column(conn: sa.engine.Connection, table: str, target_type: str) -> None:
+    """Change ``<table>.result`` to ``target_type``, carrying its default across.
+
+    PostgreSQL cannot cast an existing column default between two unrelated enum
+    types, so ALTER COLUMN ... TYPE fails outright while a default is attached.
+    Drop it, retype, then put it back -- the same order dec0deba5893 uses.
+    """
+    conn.exec_driver_sql(f"ALTER TABLE {table} ALTER COLUMN result DROP DEFAULT")
+    conn.exec_driver_sql(
+        f"ALTER TABLE {table} ALTER COLUMN result"
+        f" TYPE {target_type} USING result::text::{target_type}"
+    )
+    conn.exec_driver_sql(
+        f"ALTER TABLE {table} ALTER COLUMN result SET DEFAULT 'UNDEFINED'::{target_type}"
+    )
+
+
 def upgrade() -> None:
     conn = op.get_bind()
 
@@ -62,10 +79,7 @@ def upgrade() -> None:
                 #   - "sessionresults" was created by b6b884fbae1f (2022, sessions.result)
                 #   - ffcf0ed13a26 skipped rename because both existed
                 # Fix: alter sessions.result to use the singular type, then drop plural.
-                conn.exec_driver_sql(
-                    "ALTER TABLE sessions ALTER COLUMN result"
-                    " TYPE sessionresult USING result::text::sessionresult"
-                )
+                _retype_result_column(conn, "sessions", "sessionresult")
                 conn.exec_driver_sql("DROP TYPE sessionresults")
             elif has_plural and not has_singular:
                 # Only plural exists (ffcf0ed13a26 was never applied, or was skipped).
@@ -102,10 +116,7 @@ def downgrade() -> None:
                 conn.exec_driver_sql(
                     "CREATE TYPE sessionresults AS ENUM ('UNDEFINED', 'SUCCESS', 'FAILURE')"
                 )
-                conn.exec_driver_sql(
-                    "ALTER TABLE sessions ALTER COLUMN result"
-                    " TYPE sessionresults USING result::text::sessionresults"
-                )
+                _retype_result_column(conn, "sessions", "sessionresults")
     except sa.exc.DBAPIError as e:
         if not _is_expected_divergence(e):
             raise