Add AI Search retriever notebook

[CollierKing]

What changed

• Adds docs/ai_search.ipynb using the official Cloudflare Python SDK for AI Search instance creation, file upload, indexing status, raw search, and cleanup.
• Demonstrates CloudflareAISearchRetriever over the populated instance and shows both AsyncCloudflare and native AI Search binding usage in Python Workers.
• Uses unique resource names and deletes only the item and instance created by the notebook run.
• Uses a nonblocking upload followed by bounded polling so long indexing jobs do not exceed the SDK request timeout.

Why

The official Cloudflare Python SDK is the canonical Python administration client for AI Search. This keeps langchain-cloudflare focused on its LangChain retriever integration instead of introducing a second, partial REST administration client.

This is documentation-only and does not change package dependencies, versions, or runtime behavior.

Validation

• pre-commit run --all-files
• make lint
• make test - 136 passed, 2 skipped
• Notebook JSON and Python code-cell syntax validation
• Live lifecycle with Cloudflare Python SDK 5.5.0: create, upload, index, SDK search, LangChain retrieval, item delete, and instance delete