fix: fall back to local git watcher for chat diff drawer (#24512)

The Ctrl+D diff drawer in `coder exp agents` only rendered PR-backed
diffs returned by `/api/experimental/chats/{id}/diff`. Local working
tree changes in a chat's workspace returned an empty diff, so the
drawer showed "No diff contents" with no file summary.

Centralise diff loading behind a single `fetchChatDiffContents` helper
that first hits `/diff`, then falls back to the chat git watcher
WebSocket (`/stream/git`) when the remote diff is empty. Aggregate the
agent's `WorkspaceAgentRepoChanges` into a `ChatDiffContents` value so
the drawer can derive the file summary and styled body from the local
unified diff. Missing workspaces, missing agents, and watcher timeouts
are treated as graceful fallbacks that render the empty-diff
placeholder instead of a hard error.

> Mux is opening this PR on Mike's behalf.

codersdk/chats.go

@@ -1164,6 +1164,47 @@ type ChatDiffContents struct {
 	Diff           string    `json:"diff,omitempty"`
 }
 
+// Chat git watch error messages. These are the user-visible messages
+// the server returns in 400 responses from
+// /api/experimental/chats/{id}/stream/git when the chat cannot be
+// observed through a workspace agent. They are exported so the CLI
+// (and any future consumer) can match them structurally via
+// IsChatGitWatchFallbackMessage instead of coupling to exact wording.
+// Keep these in sync with coderd/exp_chats.go.
+const (
+	ChatGitWatchNoWorkspaceMessage       = "Chat has no workspace to watch."
+	ChatGitWatchWorkspaceNotFoundMessage = "Chat workspace not found."
+	ChatGitWatchWorkspaceNoAgentsMessage = "Chat workspace has no agents."
+	// ChatGitWatchAgentStatePrefix is the common prefix of the
+	// message produced by ChatGitWatchAgentStateMessage. The CLI
+	// uses it as a mechanical fingerprint for the "agent not yet
+	// connected" case without depending on the formatted values.
+	ChatGitWatchAgentStatePrefix = "Agent state is "
+)
+
+// ChatGitWatchAgentStateMessage is the user-visible error message
+// returned from /api/experimental/chats/{id}/stream/git when the
+// chat workspace's agent is not in the connected state.
+func ChatGitWatchAgentStateMessage(actual WorkspaceAgentStatus) string {
+	return fmt.Sprintf("%s%q, it must be in the %q state.", ChatGitWatchAgentStatePrefix, actual, WorkspaceAgentConnected)
+}
+
+// IsChatGitWatchFallbackMessage reports whether msg matches one of
+// the 400-response messages /api/experimental/chats/{id}/stream/git
+// emits when the chat cannot be observed through a workspace agent.
+// Clients should treat these cases as "no diff available" and fall
+// back to the empty remote diff instead of surfacing a hard error.
+func IsChatGitWatchFallbackMessage(msg string) bool {
+	trimmed := strings.TrimSpace(msg)
+	switch trimmed {
+	case ChatGitWatchNoWorkspaceMessage,
+		ChatGitWatchWorkspaceNotFoundMessage,
+		ChatGitWatchWorkspaceNoAgentsMessage:
+		return true
+	}
+	return strings.HasPrefix(trimmed, ChatGitWatchAgentStatePrefix)
+}
+
 // ChatStreamEventType represents the kind of chat stream update.
 type ChatStreamEventType string
 
@@ -2580,20 +2621,6 @@ func (c *ExperimentalClient) ProposeChatTitle(ctx context.Context, chatID uuid.U
 	return resp, json.NewDecoder(res.Body).Decode(&resp)
 }
 
-// GetChatGitChanges returns git changes for a chat.
-func (c *ExperimentalClient) GetChatGitChanges(ctx context.Context, chatID uuid.UUID) ([]ChatGitChange, error) {
-	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s/git-changes", chatID), nil)
-	if err != nil {
-		return nil, err
-	}
-	defer res.Body.Close()
-	if res.StatusCode != http.StatusOK {
-		return nil, ReadBodyAsError(res)
-	}
-	var changes []ChatGitChange
-	return changes, json.NewDecoder(res.Body).Decode(&changes)
-}
-
 // GetChatDiffContents returns resolved diff contents for a chat.
 func (c *ExperimentalClient) GetChatDiffContents(ctx context.Context, chatID uuid.UUID) (ChatDiffContents, error) {
 	res, err := c.Request(ctx, http.MethodGet, fmt.Sprintf("/api/experimental/chats/%s/diff", chatID), nil)