Add OAuth token fallback for provider auth

[MegaDigitalCode]

Summary

This PR adds provider credential fallback support for OAuth-style tokens in OpenFang runtime auth resolution and provider auth status detection.

What Changed

• Add env candidate resolution helper for provider credentials.
• Update driver credential resolution to support OAuth token fallbacks:
  • anthropic: ANTHROPIC_OAUTH_TOKEN then ANTHROPIC_API_KEY
  • qwen: QWEN_OAUTH_TOKEN then DASHSCOPE_API_KEY
  • minimax: MINIMAX_OAUTH_TOKEN then MINIMAX_API_KEY
• Update detect_auth() to mark providers as configured when those OAuth env vars are present.
• Update provider docs to mention OAuth token alternatives.
• Add focused tests for both runtime credential resolution and auth-status detection paths.

Why

Some providers support OAuth-derived tokens in addition to static API keys. This change aligns runtime behavior and auth-status UX with those supported auth modes.

Validation

Executed on runner environment:

• cargo test -p openfang-runtime
• Focused tests for new coverage:
  • test_detect_auth_anthropic_oauth_token
  • test_detect_auth_qwen_oauth_token
  • test_detect_auth_minimax_oauth_token
  • test_resolve_provider_api_key_prefers_anthropic_oauth
  • test_resolve_provider_api_key_qwen_oauth_fallback
  • test_resolve_provider_api_key_minimax_oauth_fallback
• cargo clippy -p openfang-runtime --all-targets -- -D warnings

Scope

Single concern only: provider auth fallback behavior plus matching docs/tests.