Return the body of a send that saved nothing

Replaces the response-directory fallback with what the frontend already
does for ephemeral sends: the engine hands the body back, because it is
the only copy. Guessing at a file named for an id was the store reaching
around its own abstraction, and it is exactly what must not survive the
move to blob storage.

The send reply carries the bytes when the engine returned them, and the
runtime holds them for the rest of the call that sent them, so
ctx.httpResponse.body() answers for a saved and an unsaved response the
same way. Unsaved ones now report a real contentType too, taken from the
response the send already handed back.

crates-cli/yaak-cli/src/plugin_events.rs

@@ -1,5 +1,7 @@
 use crate::context::CliExecutionContext;
 use arboard::Clipboard;
+use base64::Engine;
+use base64::prelude::BASE64_STANDARD;
 use console::Term;
 use inquire::{Confirm, Editor, Password, PasswordDisplayMode, Select, Text};
 use serde_json::Value;
@@ -132,7 +134,7 @@ async fn build_plugin_reply(
 
     match handle_shared_plugin_event(
         &host_context.query_manager,
-        &FileResponseBodyStore::new(&host_context.query_manager, &host_context.response_dir),
+        &FileResponseBodyStore::new(&host_context.query_manager),
         &event.payload,
         SharedPluginEventContext { plugin_name, workspace_id: shared_workspace_id },
     ) {
@@ -225,7 +227,15 @@ async fn build_plugin_reply(
                 .await
                 {
                     Ok(result) => Some(InternalEventPayload::SendHttpRequestResponse(
-                        SendHttpRequestResponse { http_response: result.response },
+                        SendHttpRequestResponse {
+                            http_response: result.response,
+                            // Nothing saved this body, so the reply is the only
+                            // place the plugin can get it.
+                            body: result
+                                .response_body
+                                .returned_bytes()
+                                .map(|b| BASE64_STANDARD.encode(b)),
+                        },
                     )),
                     Err(err) => Some(InternalEventPayload::ErrorResponse(ErrorResponse {
                         error: format!("Failed to send HTTP request in CLI: {err}"),