feat: Generate and add image captions to search index when image is ingested.

[superhindupur]

Closes #749

Purpose

This PR adds the following functionality when an image is uploaded to the knowledge base (when advanced image processing is enabled):

• generates a caption for the image by calling the OpenAI service
• generates the vector embeddings for the caption
• includes the caption text and embeddings in the document uploaded to the search index

This will allow for improved search on images.

Does this introduce a breaking change?

• Yes
• No

How to Test

Deploy Chat with your data with Advanced Image Processing enabled.
Upload an image via the admin site.
Then verify on the search index that the content and content_vector fields are populated with the appropriate caption and vector embeddings.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
code/backend/batch/utilities/helpers
env_helper.py	134	10	92%	222, 227–228, 231–233, 245, 249–251
llm_helper.py	42	11	73%	40–41, 50, 61–62, 73, 86–87, 94, 113, 121
code/backend/batch/utilities/helpers/embedders
push_embedder.py	74	0	100%
TOTAL	2434	684	71%

Tests	Skipped	Failures	Errors	Time
202	0 💤	0 ❌	0 🔥	11.817s ⏱️

[adamdougal]