feat: Proxy extensions — /v1/completions + /v1/embeddings

- Added generic execute_proxy_request executor for any path suffix
- POST /v1/completions — forwards to upstream /completions
- POST /v1/embeddings — forwards to upstream /embeddings
- Both use existing auth + route resolution infra
- 18/18 tests pass

boosapi-rs/src/proxy/executors/mod.rs

@@ -120,6 +120,47 @@ pub async fn execute_chat_completions_stream(
     Ok(chunks)
 }
 
+/// Generic non-streaming proxy request to any upstream endpoint path.
+pub async fn execute_proxy_request(
+    _state: AppState,
+    route: UpstreamRoute,
+    path_suffix: &str,
+    body: Value,
+) -> AppResult<Value> {
+    let client = http_client();
+    let upstream_url = format!(
+        "{}/{}",
+        route.base_url.trim_end_matches('/'),
+        path_suffix.trim_start_matches('/')
+    );
+
+    let resp = client
+        .post(&upstream_url)
+        .header("Authorization", format!("Bearer {}", route.api_key))
+        .header("Content-Type", "application/json")
+        .json(&body)
+        .send()
+        .await
+        .map_err(|e| AppError::UpstreamError(format!("Upstream request failed: {e}")))?;
+
+    let status = resp.status();
+    let text = resp
+        .text()
+        .await
+        .map_err(|e| AppError::UpstreamError(format!("Failed to read upstream response: {e}")))?;
+
+    if !status.is_success() {
+        return Err(AppError::UpstreamError(format!(
+            "Upstream returned {status}: {text}"
+        )));
+    }
+
+    let json: Value = serde_json::from_str(&text)
+        .map_err(|e| AppError::UpstreamError(format!("Invalid JSON from upstream: {e}")))?;
+
+    Ok(json)
+}
+
 /// Execute an image generation request.
 pub async fn execute_image_generation(
     _state: AppState,