feat: inject images via MITM layer instead of relying on LS

The LS silently ignores the 'images' field from our
SendUserCascadeMessageRequest proto — it never forwards image data
to Google's API.

New approach: store the image in MitmStore, then the MITM request
modifier injects it as 'inlineData' directly into the last user
message's parts array in the Google API JSON request.

Flow:
  Client → Proxy (decode base64) → MitmStore.set_pending_image()
  LS → Google API → MITM intercepts → inject inlineData part
  → Google receives image + text together

This works for all three API endpoints (responses, completions,
gemini).

src/mitm/proxy.rs

@@ -562,14 +562,16 @@ async fn handle_http_over_tls(
                     let pending_results = store.take_tool_results().await;
                     let last_calls = store.get_last_function_calls().await;
                     let generation_params = store.get_generation_params().await;
+                    let pending_image = store.take_pending_image().await;
 
-                    let tool_ctx = if tools.is_some() || !pending_results.is_empty() || generation_params.is_some() {
+                    let tool_ctx = if tools.is_some() || !pending_results.is_empty() || generation_params.is_some() || pending_image.is_some() {
                         Some(super::modify::ToolContext {
                             tools,
                             tool_config,
                             pending_results,
                             last_calls,
                             generation_params,
+                            pending_image,
                         })
                     } else {
                         None