Add AI Search admin client - #48

What changed

• Adds CloudflareAISearchClient for AI Search instance lifecycle, stats, uploaded item management, raw search, and chat completions.
• Supports REST usage through namespace-scoped AI Search routes, including /namespaces/default/instances/....
• Supports async Worker ai_search_namespaces bindings for namespace-level create/list/get/delete flows.
• Adds Worker example coverage with an AI_SEARCH_ADMIN namespace binding and /ai-search-admin endpoint.
• Adds docs/ai_search.ipynb with an AI Search walkthrough matching the existing notebook style.
• Adds offline unit tests, live REST lifecycle integration coverage, and focused Worker admin integration coverage.
• Hardens the shared AI Search fixture by waiting for upload completion and allowing slower indexing.
• Bumps langchain-cloudflare to 0.3.7 and adds a changelog entry.

Validation

• make test -> 148 passed, 2 skipped
• make lint -> passed
• pre-commit run --all-files -> passed
• uv build -> built langchain_cloudflare-0.3.7
• REST AI Search admin integration -> passed against /namespaces/default
• Worker AI Search focused suite -> 4 passed
• Focused Worker admin create/delete rerun -> passed
• Fresh Python 3.10 targeted AI Search unit tests -> 12 passed
• Fresh Python 3.12 targeted AI Search unit tests -> 12 passed
• docs/ai_search.ipynb JSON and code-cell syntax validation -> passed