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/db2sdk/db2sdk.go

@@ -1071,7 +1071,7 @@ func ChatMessage(m database.ChatMessage) codersdk.ChatMessage {
 		Role:          codersdk.ChatMessageRole(m.Role),
 	}
 	if m.Content.Valid {
-		parts, err := chatMessageParts(codersdk.ChatMessageRole(m.Role), m.Content)
+		parts, err := chatMessageParts(m)
 		if err == nil {
 			msg.Content = parts
 		}
@@ -1113,9 +1113,15 @@ func chatMessageUsage(m database.ChatMessage) *codersdk.ChatMessageUsage {
 
 // ChatQueuedMessage converts a queued message to its SDK representation.
 func ChatQueuedMessage(message database.ChatQueuedMessage) codersdk.ChatQueuedMessage {
-	parts, err := chatMessageParts(codersdk.ChatMessageRoleUser, pqtype.NullRawMessage{
-		RawMessage: message.Content,
-		Valid:      len(message.Content) > 0,
+	// Queued messages are always written by current code via
+	// MarshalParts, so they are always current content version.
+	parts, err := chatMessageParts(database.ChatMessage{
+		Role: database.ChatMessageRoleUser,
+		Content: pqtype.NullRawMessage{
+			RawMessage: message.Content,
+			Valid:      len(message.Content) > 0,
+		},
+		ContentVersion: chatprompt.CurrentContentVersion,
 	})
 	if err != nil {
 		parts = nil
@@ -1139,8 +1145,8 @@ func ChatQueuedMessages(messages []database.ChatQueuedMessage) []codersdk.ChatQu
 	return out
 }
 
-func chatMessageParts(role codersdk.ChatMessageRole, raw pqtype.NullRawMessage) ([]codersdk.ChatMessagePart, error) {
-	parts, err := chatprompt.ParseContent(role, raw)
+func chatMessageParts(m database.ChatMessage) ([]codersdk.ChatMessagePart, error) {
+	parts, err := chatprompt.ParseContent(m)
 	if err != nil {
 		return nil, err
 	}

coderd/database/db2sdk/db2sdk_test.go

@@ -467,7 +467,7 @@ func TestChatMessage_PreservesProviderExecutedOnToolResults(t *testing.T) {
 	dbMsg := database.ChatMessage{
 		ID:     1,
 		ChatID: uuid.New(),
-		Role:   string(codersdk.ChatMessageRoleAssistant),
+		Role:   database.ChatMessageRoleAssistant,
 		Content: pqtype.NullRawMessage{
 			RawMessage: rawContent,
 			Valid:      true,
@@ -495,8 +495,9 @@ func TestChatMessage_PreservesProviderExecutedOnToolResults(t *testing.T) {
 func TestChatQueuedMessage_ParsesUserContentParts(t *testing.T) {
 	t.Parallel()
 
-	rawContent, err := json.Marshal([]fantasy.Content{
-		fantasy.TextContent{Text: "queued text"},
+	// Queued messages are always written via MarshalParts (SDK format).
+	rawContent, err := json.Marshal([]codersdk.ChatMessagePart{
+		codersdk.ChatMessageText("queued text"),
 	})
 	require.NoError(t, err)
 
@@ -512,35 +513,15 @@ func TestChatQueuedMessage_ParsesUserContentParts(t *testing.T) {
 	require.Equal(t, "queued text", queued.Content[0].Text)
 }
 
-func TestChatQueuedMessage_FallsBackToTextForLegacyContent(t *testing.T) {
+func TestChatQueuedMessage_MalformedContent(t *testing.T) {
 	t.Parallel()
 
-	t.Run("legacy_string", func(t *testing.T) {
-		t.Parallel()
-
-		queued := db2sdk.ChatQueuedMessage(database.ChatQueuedMessage{
-			ID:        1,
-			ChatID:    uuid.New(),
-			Content:   json.RawMessage(`"legacy queued text"`),
-			CreatedAt: time.Now(),
-		})
-
-		require.Len(t, queued.Content, 1)
-		require.Equal(t, codersdk.ChatMessagePartTypeText, queued.Content[0].Type)
-		require.Equal(t, "legacy queued text", queued.Content[0].Text)
+	queued := db2sdk.ChatQueuedMessage(database.ChatQueuedMessage{
+		ID:        1,
+		ChatID:    uuid.New(),
+		Content:   json.RawMessage(`{"unexpected":"shape"}`),
+		CreatedAt: time.Now(),
 	})
 
-	t.Run("malformed_payload", func(t *testing.T) {
-		t.Parallel()
-
-		raw := json.RawMessage(`{"unexpected":"shape"}`)
-		queued := db2sdk.ChatQueuedMessage(database.ChatQueuedMessage{
-			ID:        1,
-			ChatID:    uuid.New(),
-			Content:   raw,
-			CreatedAt: time.Now(),
-		})
-
-		require.Empty(t, queued.Content)
-	})
+	require.Empty(t, queued.Content)
 }