feat: add 'copilot' to ai_provider_type (#25616)

coderd/database/migrations/000506_ai_provider_type_copilot_value.down.sql

@@ -0,0 +1,2 @@
+-- No-op: Postgres does not allow removing enum values safely.
+-- Matches the precedent in 000499_ai_provider_type_chatd_values.down.sql.

coderd/database/migrations/000506_ai_provider_type_copilot_value.up.sql

@@ -0,0 +1,5 @@
+-- Add 'copilot' to ai_provider_type. The aibridge runtime already supports
+-- Copilot via aibridge.NewCopilotProvider; the enum just needs the
+-- discriminator so DB-driven providers can carry it. Mirrors the precedent
+-- in 000499_ai_provider_type_chatd_values.up.sql.
+ALTER TYPE ai_provider_type ADD VALUE IF NOT EXISTS 'copilot';