Add MiniMax provider alias for pi and claude nodes

[octo-patch]

Reason: Add the MiniMax provider preset for Pi and Claude nodes with global OpenAI- and Anthropic-compatible endpoints.

Changes

• agentflow/specs.py: resolve the minimax provider alias in resolve_provider. For claude nodes it points at the MiniMax Anthropic-compatible endpoint (https://api.minimax.io/anthropic) with api_key_env=MINIMAX_API_KEY; for pi nodes it points at the OpenAI-compatible chat-completions endpoint (https://api.minimax.io/v1) with the same key env. The alias is rejected for codex nodes because MiniMax exposes chat completions, not the Responses API Codex requires. The China region (https://api.minimaxi.com) or any custom base URL can still be supplied via a full ProviderConfig.
• docs/pipelines.md: document the new minimax shorthand under built-in provider shorthands and note the per-agent endpoint routing, the codex rejection, and the custom-endpoint escape hatch.
• tests/test_agents.py: cover the Pi adapter materializing a scoped models.json for provider: "minimax" (baseUrl https://api.minimax.io/v1, api: openai-completions, key env MINIMAX_API_KEY, model id stripped of the minimax/ prefix) and the Claude adapter surfacing ANTHROPIC_BASE_URL=https://api.minimax.io/anthropic plus the key for the same alias.
• tests/test_store_and_validation.py: cover pipeline validation rejecting provider: "minimax" on codex nodes.

Checks

• python3 -m pytest tests/test_store_and_validation.py tests/test_agents.py tests/test_tuned_agents.py tests/test_inference.py -q -> 131 passed.
• python3 -m py_compile agentflow/specs.py tests/test_agents.py tests/test_store_and_validation.py -> OK.
• Secret scan over the diff -> no hits.

[octo-patch]

Updated the MiniMax provider coverage with global and China aliases and scoped Pi declarations for MiniMax-M3 and MiniMax-M2.7. I ran pytest tests/test_agents.py tests/test_store_and_validation.py (103 passed) and python -m compileall -q agentflow.