fix: save media message type to db (#23427)

We had a bug where computer use base64-encoded screenshots would not be
interpreted as screenshots anymore once saved to the db, loaded back
into memory, and sent to Anthropic. Instead, they would be interpreted
as regular text. Once a computer use agent made enough screenshots and
stopped, and you tried sending it another message, you'd get an out of
context error:

<img width="808" height="367" alt="Screenshot 2026-03-23 at 12 02 54"
src="https://github.com/user-attachments/assets/f0bf6be2-4863-47ca-a7a9-9e6d9dfceeed"
/>

This PR fixes that.

codersdk/chats.go

@@ -159,6 +159,7 @@ type ChatMessagePart struct {
 	Result            json.RawMessage     `json:"result,omitempty" variants:"tool-result?"`
 	ResultDelta       string              `json:"result_delta,omitempty"`
 	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?"`
 	URL               string              `json:"url" variants:"source"`
 	Title             string              `json:"title,omitempty" variants:"source?"`
@@ -241,13 +242,17 @@ func ChatMessageToolCall(toolCallID, toolName string, args json.RawMessage) Chat
 }
 
 // ChatMessageToolResult builds a tool-result chat message part.
-func ChatMessageToolResult(toolCallID, toolName string, result json.RawMessage, isError bool) ChatMessagePart {
+// The isMedia flag marks the result as carrying binary media content
+// (e.g. a screenshot) so that round-trip reconstruction preserves
+// the media type instead of sending raw base64 as text tokens.
+func ChatMessageToolResult(toolCallID, toolName string, result json.RawMessage, isError bool, isMedia bool) ChatMessagePart {
 	return ChatMessagePart{
 		Type:       ChatMessagePartTypeToolResult,
 		ToolCallID: toolCallID,
 		ToolName:   toolName,
 		Result:     result,
 		IsError:    isError,
+		IsMedia:    isMedia,
 	}
 }