Add MuAPI provider integration

[Anil-matcha]

Summary

• Add MuAPI image and video generators using MUAPI_API_KEY and the standard https://api.muapi.ai/api/v1 base URL.
• Support local reference uploads, asynchronous submit → poll handling, model overrides, and ViMax output normalization.
• Register MuAPI in the agent runtime, add ready-to-copy configs, and document TUI/Web UI setup.
• Add mocked provider tests covering success, failure, uploads, model pairing, and runtime selection.

Verification

• UV_CACHE_DIR=/private/tmp/vimax-uv-cache uv run --no-sync python -m unittest tests.test_muapi_provider -v
• UV_CACHE_DIR=/private/tmp/vimax-uv-cache uv run --no-sync python -m unittest discover -s tests
• Ruff, YAML validation, and git diff --check

The integration follows MuAPI's documented x-api-key, file upload, and submit-then-poll APIs:

• https://muapi.ai/docs/api-reference
• https://muapi.ai/docs/file-upload