Remove auto-appending provider suffixes to api_base

[Code-MonkeyZhang]

PR Description

This PR removes implicit URL rewriting in LLMClient and updates docs/templates to match.

Why

This PR trys to solve problem in issue #37. Auto-appending /anthropic or /v1 to api_base was surprising and caused real misconfigurations (e.g., users providing an OpenAI-compatible base like .../v1 ended up calling .../v1/v1 and getting 404). Requiring an explicit, provider-specific base makes behavior predictable and works better with third‑party OpenAI/Anthropic endpoints.

What changed

• LLMClient no longer appends /anthropic or /v1; it passes api_base through unchanged.
• Updated default/backup api_base values to full Anthropic-compatible bases to avoid hitting incomplete roots when config is missing fields.
• Updated config-example.yaml, README.md, and README_CN.md to remove the auto-append note and show full provider-specific api_base examples for both Anthropic and OpenAI providers.
• Breaking change / migration
  Configs that previously set api_base to a root domain (e.g., https://api.minimax.io) must now include the provider suffix

Testing

pytest tests/ -v (all unit tests pass)