feat: add config for embedding model (#12120)

This PR adds the ability to modify the embedding model used by Captain
AI.Previously, the embedding model was hardcoded which led to errors when
you used a different API provider which did not support that specific
embedding model.

Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

lib/open_ai_constants.rb

@@ -3,4 +3,5 @@
 module OpenAiConstants
   DEFAULT_MODEL = 'gpt-4.1-mini'
   DEFAULT_ENDPOINT = 'https://api.openai.com'
+  DEFAULT_EMBEDDING_MODEL = 'text-embedding-3-small'
 end