feat: add Gemini embeddings support + Letta compatibility fixes

Cherry-picked from decolua/9router#148 (author: xuandung38 / Hồ Xuân Dũng <me@hxd.vn>)

- Add Google AI (Gemini) embeddings support for /v1/embeddings endpoint
- Add Gemini embedding models: gemini-embedding-001, text-embedding-005, text-embedding-004
- Inject missing object/created fields for Letta and strict OpenAI clients
- Strip Azure-specific fields (prompt_filter_results, content_filter_results) from responses
- Fix Dockerfile: copy open-sse directory into Docker runner stage

Skipped: whitelist message field stripping (commit 3/7/8) — too aggressive for all providers
Skipped: default stream=false change (commit 9) — behavior change needs further review
Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/handlers/chatCore.js

@@ -760,6 +760,22 @@ export async function handleChatCore({ body, modelInfo, credentials, log, onCred
       ? translateNonStreamingResponse(responseBody, targetFormat, sourceFormat)
       : responseBody;
 
+    // Ensure OpenAI-required fields are present (needed for Letta and other strict clients)
+    if (!translatedResponse.object) translatedResponse.object = "chat.completion";
+    if (!translatedResponse.created) translatedResponse.created = Math.floor(Date.now() / 1000);
+
+    // Strip Azure-specific non-standard fields
+    if (translatedResponse.prompt_filter_results !== undefined) {
+      delete translatedResponse.prompt_filter_results;
+    }
+    if (translatedResponse?.choices) {
+      for (const choice of translatedResponse.choices) {
+        if (choice.content_filter_results !== undefined) {
+          delete choice.content_filter_results;
+        }
+      }
+    }
+
     // Add buffer and filter usage for client (to prevent CLI context errors)
     if (translatedResponse?.usage) {
       const buffered = addBufferToUsage(translatedResponse.usage);