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;

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

@@ -0,0 +1,19 @@
+CREATE TYPE aibridge_interception_error_type AS ENUM (
+    'bad_request',
+    'unauthorized',
+    'rate_limited',
+    'overloaded',
+    'server_error',
+    'timeout',
+    'unknown'
+);
+
+-- Records the terminal upstream error observed when an interception failed.
+-- Both columns are NULL for interceptions that completed successfully.
+-- error_message is capped at 1024 characters as a hard schema-level bound.
+ALTER TABLE aibridge_interceptions
+    ADD COLUMN error_type aibridge_interception_error_type,
+    ADD COLUMN error_message varchar(1024);
+
+COMMENT ON COLUMN aibridge_interceptions.error_type IS 'Categorised terminal upstream error for a failed interception; NULL when the interception succeeded.';
+COMMENT ON COLUMN aibridge_interceptions.error_message IS 'Raw terminal upstream error message for a failed interception; NULL when the interception succeeded.';