INTPYTHON-881 Add auto-embedding to retrievers (#314)

Patch build:
https://spruce.mongodb.com/version/698386a6967f4f000735585f/tasks?sorts=STATUS%3AASC%3BBASE_STATUS%3ADESC

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INTPYTHON-881

## Summary

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the JIRA ticket, still place it in the Pull Request as you should not
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Add auto-embedding support to retrievers.

## Changes in this PR

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private) were added, removed, or edited to generate the desired outcome
explained in the above summary? -->

Conditionally use auto-embedding support added in #305 to retrievers.

## Test Plan

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that. If you didn’t introduce unit tests, explain why. All code should
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Retrievers in langchain-mongodb:

1. ✅ MongoDBAtlasHybridSearchRetriever - auto-embedding support added
        • Implementation: Lines 83-136 in hybrid_search.py
• Checks if using AutoEmbeddings and uses
autoembedding_vector_search_stage instead of vector_search_stage
• Test: test_hybrid_retriever_autoembed in test_retrievers.py (lines
253-270)

2. ✅ MongoDBAtlasParentDocumentRetriever - auto-embedding support added
        • Implementation: Lines 86-116 in parent_document.py
• Checks if using AutoEmbeddings and uses
autoembedding_vector_search_stage instead of vector_search_stage
• Test: test_1clxn_retriever with parametrized embedding (auto/manual)
in test_parent_document.py (lines 42-110)

3. ✅ MongoDBAtlasSelfQueryRetriever - auto-embedding support inherited
        • This retriever wraps a MongoDBAtlasVectorSearch vectorstore


## Checklist

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### Checklist for Author

- [x] Did you update the changelog (if necessary)?
- [x] Is the intention of the code captured in relevant tests?
- [ ] If there are new TODOs, has a related JIRA ticket been created?
- [x] Has a MongoDB Employee run [the patch build of this
PR](https://github.com/mongodb-labs/ai-ml-pipeline-testing?tab=readme-ov-file#running-a-patch-build-of-a-given-pr)?

### Checklist for Reviewer

- [ ] Does the title of the PR reference a JIRA Ticket?
- [ ] Do you fully understand the implementation? (Would you be
comfortable explaining how this code works to someone else?)
- [ ] Is all relevant documentation (README or docstring) updated?

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Co-authored-by: Steven Silvester <steve.silvester@mongodb.com>