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**

provisioner/terraform/testdata/resources/devcontainer/converted_state.state.golden

@@ -24,15 +24,13 @@
             {
               "workspace_folder": "/workspace1",
               "name": "dev1",
-              "id": "eb9b7f18-c277-48af-af7c-2a8e5fb42bab",
-              "subagent_id": "72d17819-ea3b-450a-a502-175886583ecf"
+              "id": "eb9b7f18-c277-48af-af7c-2a8e5fb42bab"
             },
             {
               "workspace_folder": "/workspace2",
               "config_path": "/workspace2/.devcontainer/devcontainer.json",
               "name": "dev2",
-              "id": "964430ff-f0d9-4fcb-b645-6333cf6ba9f2",
-              "subagent_id": "40a59d56-d3df-488f-b07d-331c0b774bac"
+              "id": "964430ff-f0d9-4fcb-b645-6333cf6ba9f2"
             }
           ],
           "api_key_scope": "all"

provisioner/terraform/testdata/resources/devcontainer/devcontainer.tfplan.json

@@ -139,8 +139,7 @@
         },
         "after_unknown": {
           "agent_id": true,
-          "id": true,
-          "subagent_id": true
+          "id": true
         },
         "before_sensitive": false,
         "after_sensitive": {}
@@ -163,8 +162,7 @@
         },
         "after_unknown": {
           "agent_id": true,
-          "id": true,
-          "subagent_id": true
+          "id": true
         },
         "before_sensitive": false,
         "after_sensitive": {}

provisioner/terraform/testdata/resources/devcontainer/devcontainer.tfstate.json

@@ -60,7 +60,6 @@
             "agent_id": "eb1fa705-34c6-405b-a2ec-70e4efd1614e",
             "config_path": null,
             "id": "eb9b7f18-c277-48af-af7c-2a8e5fb42bab",
-            "subagent_id": "72d17819-ea3b-450a-a502-175886583ecf",
             "workspace_folder": "/workspace1"
           },
           "sensitive_values": {},
@@ -79,7 +78,6 @@
             "agent_id": "eb1fa705-34c6-405b-a2ec-70e4efd1614e",
             "config_path": "/workspace2/.devcontainer/devcontainer.json",
             "id": "964430ff-f0d9-4fcb-b645-6333cf6ba9f2",
-            "subagent_id": "40a59d56-d3df-488f-b07d-331c0b774bac",
             "workspace_folder": "/workspace2"
           },
           "sensitive_values": {},