okf: add MiniMax provider (MiniMax-M3, MiniMax-M2.7) to the reference agent

[octo-patch]

Reason: The reference agent had no MiniMax provider, so MiniMax-M3 / MiniMax-M2.7 and the global and China endpoints could not be selected.

What this adds

The reference agent threaded a single model string straight into
google.adk.Agent and the index synthesizer's google.genai client, so the
only reachable backend was the default Gemini one. This adds MiniMax as a second
provider without changing that default.

• New reference_agent.providers module holding a small MiniMax registry:
  • Models: MiniMax-M3 (1,000,000-token context; text/image/video input;
    adaptive/disabled thinking) and MiniMax-M2.7 (204,800-token context;
    text input; always-on thinking), each with per-million-token pricing.
  • Two regional endpoints: global_en (https://api.minimax.io/v1) and
    cn_zh (https://api.minimaxi.com/v1), each with its chat and Anthropic
    base URLs and docs root. The region is chosen with KC_MINIMAX_REGION
    (default global_en).
• build_model() returns a LiteLlm bound to the selected endpoint when the
  --model value names a MiniMax model, and returns the string unchanged
  otherwise — so a Gemini id keeps the existing behavior byte for byte.
• agent.py (both the BQ and web agents) and the index synthesizer route
  through the resolver; a MiniMax model reads its key from MINIMAX_API_KEY.
• --model help and the README credentials section document the new provider,
  the two models, the endpoint selection, and the API-key variable.

Heavy imports (google.adk LiteLlm, litellm) are lazy, only loaded when a
MiniMax model is actually used, so the default path takes on no new dependency.

Checks

• python3 -m pytest -q tests/test_providers.py tests/test_index.py tests/test_bundle_tools.py tests/test_viewer.py tests/test_document.py — 44 passed. New tests/test_providers.py covers model detection, both model specs, both regional endpoints, region override/validation, and the pass-through of non-MiniMax names.
• ruff check --select I,F,E on the changed source files — clean.

The remaining tests in this suite require optional runtime dependencies
(google-adk, google-cloud-bigquery, markdownify) that are not part of this
change and were not installable in the sandbox.