Add MiniMax provider support

[octo-patch]

Reason: add target provider/model to existing provider registry

Summary

• Add minimax to the supported provider registry and configuration, including MINIMAX_API_KEY and configurable OpenAI-compatible base URL defaults.
• Route MiniMax through ChatOpenAI with the target MiniMax-M3 model documented in provider metadata and examples.
• Update provider documentation and README compatibility tables to include MiniMax.

Checks

• git add -A -N && if git diff HEAD | grep -E "$SECRET_RE"; then echo secret_scan_failed; exit 1; fi
• python3 -m compileall -q src/agentic_architectures/config.py src/agentic_architectures/llm/factory.py src/agentic_architectures/llm/providers.py
• git diff --check

Note: python3 -m ruff check src/agentic_architectures/config.py src/agentic_architectures/llm/factory.py src/agentic_architectures/llm/providers.py could not run because ruff is not installed in the environment.

[octo-patch]

This PR is superseded by #15, which keeps the MiniMax provider change on a clean branch and includes both supported models, all four endpoint configurations, and request-path tests.