refactor: add chat_message_role enum and content_version column (#23042)

Migration 000434 converts chat_messages.role from text to a Postgres
enum, rebuilds the partial index, and adds content_version smallint.
The column is backfilled with DEFAULT 0, then the default is dropped
so future inserts must set it explicitly.

Version 0 uses the role-aware heuristic from #22958. Version 1 (all
new inserts) stores []ChatMessagePart JSON for all roles, including
system messages. ParseContent takes database.ChatMessage directly
and dispatches on version internally. Unknown versions error.

All string(codersdk.ChatMessageRole*) casts at DB write sites are
replaced with database.ChatMessageRole* constants from sqlc.

Refs #22958

coderd/database/migrations/000434_chat_message_role_and_content_version.down.sql

@@ -0,0 +1,15 @@
+ALTER TABLE chat_messages DROP COLUMN content_version;
+
+DROP INDEX idx_chat_messages_compressed_summary_boundary;
+
+ALTER TABLE chat_messages
+	ALTER COLUMN role TYPE text
+		USING (role::text);
+
+CREATE INDEX idx_chat_messages_compressed_summary_boundary
+	ON chat_messages(chat_id, created_at DESC, id DESC)
+	WHERE compressed = TRUE
+		AND role = 'system'
+		AND visibility IN ('model', 'both');
+
+DROP TYPE chat_message_role;

coderd/database/migrations/000434_chat_message_role_and_content_version.up.sql

@@ -0,0 +1,32 @@
+-- Add chat_message_role enum.
+CREATE TYPE chat_message_role AS ENUM (
+	'system',
+	'user',
+	'assistant',
+	'tool'
+);
+
+-- Drop the partial index that references role as text before
+-- converting the column type.
+DROP INDEX idx_chat_messages_compressed_summary_boundary;
+
+-- Convert role column from text to enum.
+ALTER TABLE chat_messages
+	ALTER COLUMN role TYPE chat_message_role
+		USING (role::chat_message_role);
+
+-- Recreate the partial index with enum-typed comparison.
+CREATE INDEX idx_chat_messages_compressed_summary_boundary
+	ON chat_messages(chat_id, created_at DESC, id DESC)
+	WHERE compressed = TRUE
+		AND role = 'system'
+		AND visibility IN ('model', 'both');
+
+-- Add content_version column. Default 0 backfills existing rows.
+-- The default is then dropped so future inserts must specify the
+-- version explicitly.
+ALTER TABLE chat_messages
+	ADD COLUMN content_version smallint NOT NULL DEFAULT 0;
+
+ALTER TABLE chat_messages
+	ALTER COLUMN content_version DROP DEFAULT;