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/config/providerModels.js

@@ -135,6 +135,10 @@ export const PROVIDER_MODELS = {
     { id: "gemini-2.5-pro", name: "Gemini 2.5 Pro" },
     { id: "gemini-2.5-flash", name: "Gemini 2.5 Flash" },
     { id: "gemini-2.5-flash-lite", name: "Gemini 2.5 Flash Lite" },
+    // Embedding models
+    { id: "gemini-embedding-001", name: "Gemini Embedding 001", type: "embedding" },
+    { id: "text-embedding-005", name: "Text Embedding 005", type: "embedding" },
+    { id: "text-embedding-004", name: "Text Embedding 004 (Legacy)", type: "embedding" },
   ],
   openrouter: [
     { id: "auto", name: "Auto (Best Available)" },