feat(providers): self-hosted OpenAI-compatible STT, TTS and embedding providers

Add Self-hosted STT/TTS/Embedding providers that read baseUrl per connection
instead of a fixed registry endpoint, so 9Router can point at whisper.cpp,
faster-whisper, Kokoro-FastAPI, llama-server, vLLM, Infinity, and similar
OpenAI-compatible local servers.

Self-hosted Embedding refuses to run without a baseUrl rather than falling
back to api.openai.com like openaiCompatNode does, since that fallback would
silently send input text and the API key to OpenAI under a provider named
"Self-hosted". Also fixes embeddingsCore to catch adapter build errors as a
400 instead of letting them escape uncaught, and bounds the upstream fetch
with FETCH_CONNECT_TIMEOUT_MS to avoid hanging forever on a dead endpoint.

Self-hosted TTS treats a bare model value as the model rather than the voice,
since the generic OpenAI TTS convention (bare = voice) is backwards for a
provider where the model is the variable part.

open-sse/handlers/embeddingProviders/index.js

@@ -2,6 +2,7 @@
 import createOpenAIEmbeddingAdapter from "./openai.js";
 import gemini from "./gemini.js";
 import openaiCompatNode from "./openaiCompatNode.js";
+import selfhostedEmbedding from "./selfhostedEmbedding.js";
 
 const OPENAI_COMPAT_PROVIDERS = [
   "openai", "openrouter", "mistral", "voyage-ai", "fireworks",
@@ -13,6 +14,12 @@ const ADAPTERS = {
   ...Object.fromEntries(OPENAI_COMPAT_PROVIDERS.map((id) => [id, createOpenAIEmbeddingAdapter(id)])),
   gemini,
   google_ai_studio: gemini,
+  // Self-hosted reads creds.providerSpecificData.baseUrl (one provider, many
+  // servers) — but via its OWN adapter, not openaiCompatNode: that one falls back
+  // to api.openai.com when no baseUrl is set, which under a provider called
+  // "Self-hosted Embedding" means silently shipping the input and API key to
+  // OpenAI. selfhostedEmbedding refuses instead.
+  "selfhosted-embedding": selfhostedEmbedding,
 };
 
 export function getEmbeddingAdapter(provider) {