feat(coderd/database): add error columns to aibridge interception records (#26960)

Adds a nullable `aibridge_interception_error_type` enum and an
`error_message` column to `aibridge_interceptions`, so a failed
interception's terminal upstream error can be persisted.

Schema only: the write path and API exposure land in the stacked
backend PR.

*This PR was produced by opencode (agent) using the `anthropic/claude-opus-4-8` model, under human direction and review.*

coderd/database/migrations/000541_aibridge_interception_error.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE aibridge_interceptions
+    DROP COLUMN error_type,
+    DROP COLUMN error_message;
+
+DROP TYPE aibridge_interception_error_type;