feat: stream advisor tool output (#25032)

Stream advisor output into the advisor tool card while the nested
advisor call is still running.

This keeps the advisor implementation intentionally advisor-specific:
the parent model still receives the same final structured tool result,
while the frontend receives transient `tool-result.result_delta` parts
to render partial advisor text in the expanded card. The final persisted
chat history remains unchanged.

Refs CODAGT-322.

Generated by Coder Agents.

<details>
<summary>Implementation plan</summary>

- Publish advisor text deltas from the nested `chatloop.Run` via
`RunAdvisorOptions.OnAdviceDelta`.
- Forward those deltas through `chatadvisor.Tool` with the parent
advisor tool call ID.
- Emit transient `ChatMessagePartTypeToolResult` websocket parts with
`ResultDelta` from `chatd`.
- Add `result_delta` to the generated tool-result TypeScript variant.
- Accumulate tool result deltas in frontend stream state and keep the
tool running until the final result arrives.
- Render streamed advisor advice in the existing advisor card using
streaming markdown mode, while retaining the updated advisor UI.

</details>

codersdk/chats.go

@@ -255,7 +255,8 @@ type ChatMessagePart struct {
 	Args              json.RawMessage     `json:"args,omitempty" variants:"tool-call?"`
 	ArgsDelta         string              `json:"args_delta,omitempty" variants:"tool-call?"`
 	Result            json.RawMessage     `json:"result,omitempty" variants:"tool-result?"`
-	ResultDelta       string              `json:"result_delta,omitempty"`
+	ResultDelta       string              `json:"result_delta,omitempty" variants:"tool-result?"`
+	ResultReset       bool                `json:"result_reset,omitempty" variants:"tool-result?"`
 	IsError           bool                `json:"is_error,omitempty" variants:"tool-result?"`
 	IsMedia           bool                `json:"is_media,omitempty" variants:"tool-result?"`
 	SourceID          string              `json:"source_id,omitempty" variants:"source?"`
@@ -327,9 +328,11 @@ type ChatMessagePart struct {
 // StripInternal removes internal-only fields that must not be
 // sent to API clients. Call before publishing via REST or SSE.
 //
-// Note: ArgsDelta and ResultDelta are intentionally preserved.
-// They are streaming-only fields consumed by the frontend via
-// SSE message_part events (see processStepStream in chatloop).
+// Note: ArgsDelta, ResultDelta, and ResultReset are intentionally preserved.
+// They are streaming-only fields consumed by the frontend via SSE
+// message_part events. ArgsDelta is produced by processStepStream in
+// chatloop; ResultDelta and ResultReset are produced by the advisor
+// streaming callbacks in chatd.
 func (p *ChatMessagePart) StripInternal() {
 	p.ProviderMetadata = nil
 	if p.FileID.Valid {

codersdk/chats_test.go

@@ -264,7 +264,6 @@ func TestChatMessagePartVariantTags(t *testing.T) {
 	excludedFields := map[string]string{
 		"type":                         "discriminant, added automatically by codegen",
 		"signature":                    "added in #22290, never populated by any code path",
-		"result_delta":                 "added in #22290, never populated by any code path",
 		"provider_metadata":            "internal only, stripped by db2sdk before API responses",
 		"context_file_content":         "internal only, stripped before API responses (typescript:\"-\")",
 		"context_file_os":              "internal only, used during prompt expansion (typescript:\"-\")",