refactor: remove reasoning title extraction from chat pipeline (#22926)

Removes the backend and frontend logic that extracted compact titles
from reasoning/thinking blocks. The `Title` field on `ChatMessagePart`
remains for other part types (e.g. source), but reasoning blocks no
longer have titles derived from first-line markdown bold text or
provider metadata summaries.

**Backend:**
- Remove `ReasoningTitleFromFirstLine`, `reasoningTitleFromContent`,
`reasoningSummaryTitle`, `compactReasoningSummaryTitle`, and
`reasoningSummaryHeadline` from chatprompt
- Simplify `marshalContentBlock` to plain `json.Marshal` (no title
injection)
- Remove title tracking maps and `setReasoningTitleFromText` from
chatloop stream processing
- Remove `reasoningStoredTitle` from db2sdk
- Remove related tests from db2sdk_test

**Frontend:**
- Remove `mergeThinkingTitles` from blockUtils
- Simplify `appendTextBlock` to always merge consecutive thinking blocks
- Remove `applyStreamThinkingTitle` from streamState
- Simplify reasoning/thinking stream handler to ignore title-only parts
- Update tests accordingly

Net: **-487 lines / +42 lines**

coderd/database/db2sdk/db2sdk.go

@@ -1165,10 +1165,7 @@ func chatMessageParts(role string, raw pqtype.NullRawMessage) ([]codersdk.ChatMe
 				continue
 			}
 			if i < len(rawBlocks) {
-				switch part.Type {
-				case codersdk.ChatMessagePartTypeReasoning:
-					part.Title = reasoningStoredTitle(rawBlocks[i])
-				case codersdk.ChatMessagePartTypeFile:
+				if part.Type == codersdk.ChatMessagePartTypeFile {
 					if fid, err := chatprompt.ExtractFileID(rawBlocks[i]); err == nil {
 						part.FileID = uuid.NullUUID{UUID: fid, Valid: true}
 					}
@@ -1267,22 +1264,6 @@ func parseToolResults(raw pqtype.NullRawMessage) ([]toolResultRow, error) {
 	return results, nil
 }
 
-func reasoningStoredTitle(raw json.RawMessage) string {
-	var envelope struct {
-		Type string `json:"type"`
-		Data struct {
-			Title string `json:"title"`
-		} `json:"data"`
-	}
-	if err := json.Unmarshal(raw, &envelope); err != nil {
-		return ""
-	}
-	if !strings.EqualFold(envelope.Type, string(fantasy.ContentTypeReasoning)) {
-		return ""
-	}
-	return strings.TrimSpace(envelope.Data.Title)
-}
-
 func contentBlockToPart(block fantasy.Content) codersdk.ChatMessagePart {
 	switch value := block.(type) {
 	case fantasy.TextContent:

coderd/database/db2sdk/db2sdk_test.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"charm.land/fantasy"
-	fantasyopenai "charm.land/fantasy/providers/openai"
 	"github.com/google/uuid"
 	"github.com/sqlc-dev/pqtype"
 	"github.com/stretchr/testify/require"
@@ -438,82 +437,6 @@ func TestAIBridgeInterception(t *testing.T) {
 	}
 }
 
-func TestChatMessage_ReasoningPartWithoutPersistedTitleIsEmpty(t *testing.T) {
-	t.Parallel()
-
-	assistantContent, err := json.Marshal([]fantasy.Content{
-		fantasy.ReasoningContent{
-			Text: "Plan migration",
-			ProviderMetadata: fantasy.ProviderMetadata{
-				fantasyopenai.Name: &fantasyopenai.ResponsesReasoningMetadata{
-					ItemID:  "reasoning-1",
-					Summary: []string{"Plan migration"},
-				},
-			},
-		},
-	})
-	require.NoError(t, err)
-
-	message := db2sdk.ChatMessage(database.ChatMessage{
-		ID:        1,
-		ChatID:    uuid.New(),
-		CreatedAt: time.Now(),
-		Role:      string(fantasy.MessageRoleAssistant),
-		Content: pqtype.NullRawMessage{
-			RawMessage: assistantContent,
-			Valid:      true,
-		},
-	})
-
-	require.Len(t, message.Content, 1)
-	require.Equal(t, codersdk.ChatMessagePartTypeReasoning, message.Content[0].Type)
-	require.Equal(t, "Plan migration", message.Content[0].Text)
-	require.Empty(t, message.Content[0].Title)
-}
-
-func TestChatMessage_ReasoningPartPrefersPersistedTitle(t *testing.T) {
-	t.Parallel()
-
-	reasoningContent, err := json.Marshal(fantasy.ReasoningContent{
-		Text: "Verify schema updates, then apply changes in order.",
-		ProviderMetadata: fantasy.ProviderMetadata{
-			fantasyopenai.Name: &fantasyopenai.ResponsesReasoningMetadata{
-				ItemID: "reasoning-1",
-				Summary: []string{
-					"**Metadata-derived title**\n\nLonger explanation.",
-				},
-			},
-		},
-	})
-	require.NoError(t, err)
-
-	var envelope map[string]any
-	require.NoError(t, json.Unmarshal(reasoningContent, &envelope))
-	dataValue, ok := envelope["data"].(map[string]any)
-	require.True(t, ok)
-	dataValue["title"] = "Persisted stream title"
-
-	encodedReasoning, err := json.Marshal(envelope)
-	require.NoError(t, err)
-	assistantContent, err := json.Marshal([]json.RawMessage{encodedReasoning})
-	require.NoError(t, err)
-
-	message := db2sdk.ChatMessage(database.ChatMessage{
-		ID:        1,
-		ChatID:    uuid.New(),
-		CreatedAt: time.Now(),
-		Role:      string(fantasy.MessageRoleAssistant),
-		Content: pqtype.NullRawMessage{
-			RawMessage: assistantContent,
-			Valid:      true,
-		},
-	})
-
-	require.Len(t, message.Content, 1)
-	require.Equal(t, codersdk.ChatMessagePartTypeReasoning, message.Content[0].Type)
-	require.Equal(t, "Persisted stream title", message.Content[0].Title)
-}
-
 func TestChatQueuedMessage_ParsesUserContentParts(t *testing.T) {
 	t.Parallel()