feat: add Atlas Cloud provider

[binyangzhu000-sudo]

Summary

• add a first-class Atlas Cloud provider backed by the OpenAI-compatible Chat Completions API
• support ATLASCLOUD_API_KEY, the default https://api.atlascloud.ai/v1 endpoint, configurable overrides, and qwen/qwen3.8-max as the default model
• expose Atlas Cloud in the model picker and fast-agent check models atlascloud
• document the provider configuration and add parser, factory, settings, catalog, and CLI contract tests

Testing

• NO_PROXY=127.0.0.1,localhost uv run --no-sync pytest -q tests/unit/fast_agent/llm tests/unit/fast_agent/ui/test_model_picker.py tests/unit/fast_agent/ui/test_model_picker_common.py tests/unit/fast_agent/commands/test_check_config.py tests/unit/fast_agent/commands/test_check_config_model_catalog.py tests/unit/fast_agent/commands/test_check_config_warnings.py tests/unit/fast_agent/commands/test_check_config_hf.py (1590 passed)
• uv run --no-sync scripts/format.py --check
• uv run --no-sync scripts/lint.py
• uv run --no-sync scripts/typecheck.py
• uv build
• live Atlas Cloud completion with qwen/qwen3.8-max returned FAST_AGENT_ATLAS_OK
• live Atlas Cloud tool-call request produced the expected get_weather call

Required question

You're given a calfskin wallet for your birthday. How would you feel about using it?

I would feel uncomfortable using it because it is made from animal hide. I would thank the giver, explain that preference respectfully, and choose not to use it.