feat(claude): add llm-gateway authent capability

[lindmin]

Adds support to claude for llmgateway auth e.g LiteLLM via the env vars:
ANTHROPIC_BASE_URL & ANTHROPIC_AUTH_TOKEN

ref: https://code.claude.com/docs/en/llm-gateway

information: claude code sonnet 4.6 used for this PR.
i'm not familiar to PR on opensource project, I appreciate feedback :)

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[gemini-code-assist]

Code Review

This pull request introduces support for an LLM gateway (proxy) for Anthropic models, enabling the use of tools like LiteLLM. It adds AnthropicAuthToken and AnthropicBaseURL to the authentication configuration, updates environment variable handling, and implements the necessary logic for the llm-gateway authentication method across various harness components. Review feedback suggests refining the auto-detection logic to trigger primarily on the presence of ANTHROPIC_BASE_URL. This change would prevent false positives and avoid fatal errors in environments where ANTHROPIC_AUTH_TOKEN might be set for unrelated purposes.