refactor(llm): extract shared http_client sync/async split helper

[matteomedioli]

Description

Stacked on top of #565. While working on that fix, we noticed the http_client sync/async type-routing logic — pop http_client from kwargs, route an httpx.Client to the sync SDK client, an httpx.AsyncClient to the async one, warn and drop anything else — is copy-pasted verbatim three times: AnthropicLLM.__init__, OpenAILLM.__init__, and AzureOpenAILLM.__init__.

Since BaseAnthropicLLM/BaseOpenAILLM don't do any SDK client construction themselves (by design — that's left entirely to subclasses), any future subclass built to reach a custom endpoint would otherwise become a 4th/5th copy of this exact logic. Extracting it into one shared helper now means new subclasses inherit the fix instead of copying it.

Changes:

• Added neo4j_graphrag.llm.utils.split_http_client_kwargs(kwargs) -> (sync_kwargs, async_kwargs), a single implementation of the pop/route/warn logic.
• AnthropicLLM.__init__, OpenAILLM.__init__, and AzureOpenAILLM.__init__ now all call this helper instead of repeating the isinstance checks and warning inline.
• Removed the now-unused httpx/warnings imports from anthropic_llm.py/openai_llm.py where they were only present for this logic.
• Added direct unit tests for split_http_client_kwargs in tests/unit/llm/test_llm_utils.py (no-client passthrough, sync routing, async routing, invalid-type warn-and-drop, and non-mutation of the caller's input dict) — previously this logic only had indirect coverage through each provider class's own constructor tests.

Pure refactor: no behavior change for any existing caller.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

(Refactor, no behavior change — none of the boxes above are an exact fit; flagging explicitly since the template has no "refactor" option.)

Complexity

Complexity: Low

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

New tests/unit/llm/test_llm_utils.py cases directly exercise split_http_client_kwargs: no http_client passed, httpx.Client routed to sync only, httpx.AsyncClient routed to async only, invalid type warns and is dropped from both, input dict is not mutated. Full existing tests/unit/llm/ suite passes unchanged. mypy/ruff check/ruff format --check all clean.

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate