Add PyMongo + Document DB Playground

[hossain-rayhan]

Summary

Adds a new PyMongo playground under playgrounds/pymongo/, following the same structure as the existing Mongoose and Beanie playgrounds. Unlike those ODM-based demos, this one exercises DocumentDB at the raw driver level (collections + BSON documents, no ODM), paired with Flask as the synchronous web framework.

What's Included

• Flask + PyMongo REST API (app/main.py): /health, POST/GET /books, GET/PATCH/DELETE /books/{id}, GET /stats/genres.
• Connection helpers (app/db.py): MongoClient with DocumentDB's required options (directConnection, tls, tlsAllowInvalidCertificates) and automatic replicaSet=rs0 stripping.
• Collection helpers (app/models/book.py): collection name, index specs (no collation), document builder + JSON serializer.
• Standalone CRUD/compatibility suite (app/pymongo_crud_test.py): 16 steps covering connect, index creation, insert, query, update, aggregation, unique-index enforcement, delete, and vector search (cosmosSearch vector-ivf index + $vectorSearch nearest-neighbor query).
• Scripts (scripts/): lib.sh, start-documentdb.sh, stop-documentdb.sh, run-test.sh, run-app.sh — auto-start DocumentDB if needed, KEEP_DB support, shared credentials aligned with the other playgrounds.
• README documenting architecture, quick start, connection notes, configuration reference, compatibility table, and troubleshooting.

Repo-Level Changes

• Linked the PyMongo playground in the top-level README.md (playground table, getting-started commands, repository layout).
• .gitignore already covers the Python artifacts (.venv/, __pycache__/, *.pyc) — no change needed.

Validation

• CRUD/compatibility suite: Passed 16 / Failed 0 (including vector search).
• API smoke tests passed: GET /health, POST /books (201), GET /books/{id}, PATCH /books/{id}, GET /books, GET /stats/genres, DELETE /books/{id} (204).
• Shell scripts pass bash -n syntax checks.

Notes

• PyMongo is synchronous, so this playground uses Flask (the natural sync counterpart to Express/FastAPI in the sibling playgrounds).
• No generated artifacts committed (.venv, __pycache__ removed); DocumentDB container stopped after verification.