fix(llm): wire base_url into AnthropicLLM and ZaiLLM clients

[lightzt99]

Summary

• AnthropicConfig already declares anthropic_base_url (env: ANTHROPIC_LLM_BASE_URL) but
  AnthropicLLM.__init__ never forwarded it to the anthropic.Anthropic() constructor — the field
  was silently ignored.
• Added a secondary fallback to ANTHROPIC_BASE_URL so users whose Claude Code sets that variable
  automatically get the correct endpoint without extra .env config.
• ZaiConfig already declares zai_base_url (env: ZAI_BASE_URL, default
  https://open.bigmodel.cn/api/paas/v4/) but ZaiLLM.__init__ never forwarded it either.
• Replaced the ZhipuAiClient (from the zai SDK) with openai.OpenAI since Zhipu AI exposes
  an OpenAI-compatible endpoint — base_url is then passed directly, and a separate SDK
  dependency is removed with no behaviour change.
• Added unit tests for both providers following the pattern of tests/unit/test_openai.py.

Motivation

Users pointing AnthropicLLM or ZaiLLM at a private proxy or a regional mirror had no
working way to redirect requests even though the config fields existed:

Provider	Config field	Env var	Effect before this fix
Anthropic	anthropic_base_url	ANTHROPIC_LLM_BASE_URL	Declared but never read — every request went to api.anthropic.com
Anthropic	—	ANTHROPIC_BASE_URL	Not read at all — Claude Code users needed manual workaround
ZAI	zai_base_url	ZAI_BASE_URL	Declared but never read — always hit the hardcoded Zhipu default

Changes

File	What changed
src/powermem/integrations/llm/anthropic.py	Read anthropic_base_url from config and ANTHROPIC_BASE_URL from env; pass base_url to anthropic.Anthropic()
src/powermem/integrations/llm/zai.py	Replace ZhipuAiClient with openai.OpenAI; read zai_base_url from config and pass as base_url
tests/unit/test_anthropic.py	New — covers config field, ANTHROPIC_LLM_BASE_URL, and ANTHROPIC_BASE_URL
tests/unit/test_zai.py	New — covers default URL, config field, ZAI_BASE_URL, generate_response, callback

Test plan

• pytest tests/unit/test_anthropic.py tests/unit/test_zai.py — 6 passed
• Real ZAI API call with zai_base_url explicitly set — response returned correctly
• Real ZAI API call with default zai_base_url — response returned correctly
• AnthropicLLM with anthropic_base_url config field — client.base_url matches
• AnthropicLLM with ANTHROPIC_BASE_URL env var — client.base_url matches