Add OrcaRouter as a named provider

[XiaoHuo888-hue]

Add OrcaRouter as a named provider

This registers OrcaRouter the same way the existing openrouter provider is wired, so the provider picker, config reference and docs stay consistent. OrcaRouter is an OpenAI-compatible gateway: one ORCAROUTER_API_KEY (keys start with sk-orca-) unlocks 150+ models from OpenAI, Anthropic, Google, DeepSeek, Qwen, MiniMax and xAI behind a single https://api.orcarouter.ai/v1 endpoint. It also runs gateway-level, zero-trust security for AI agents on the same endpoint — screening every prompt/response and governing every tool call on a default-deny basis, with no application code changes.

I'm an engineer on the OrcaRouter team.

Changes

• src/mcp_cli/model_management/gateway_providers.py (new) — built-in gateway provider definitions, registered into chuk_llm through its public register_provider() API at startup. Registration is idempotent: if a future chuk-llm release ships orcarouter natively, mcp-cli leaves it untouched.
• src/mcp_cli/model_management/model_manager.py — call register_gateway_providers() when initializing the chuk_llm config manager, so orcarouter shows up in /providers list out of the box.
• src/mcp_cli/auth/provider_tokens.py — map orcarouter → ORCAROUTER_API_KEY in PROVIDER_ENV_VAR_MAP (first-class status alongside openrouter).
• README.md — document the provider in the provider table, the env var in prerequisites, and add a /provider orcarouter example.
• tests/model_management/test_gateway_providers.py (new) — unit tests for registration behavior and the env var mapping.

Default model is openai/gpt-4o-mini (a fixed, tool-calling-safe model) rather than the smart router orcarouter/auto, so agentic/tool workflows are reliable out of the box.

Verification

• pytest tests/model_management tests/auth — 176 passed (includes 6 new gateway-provider tests)
• pytest tests/llm tests/config tests/cli — 446 passed, 8 skipped; the 8 failures in tests/cli/test_main_coverage.py / test_run_command_extended.py are pre-existing on main (confirmed via clean-checkout repro, unrelated to this change)
• mypy on changed modules — no issues
• ruff format --check — clean (the only ruff check findings are pre-existing BLE001/I001/SIM118/UP037 in unchanged code)
• Live test against the real OrcaRouter gateway through the registered provider path: get_client('orcarouter', 'openai/gpt-4o-mini') → 200, PONG reply; ModelManager surfaces orcarouter (16 providers) and returns openai/gpt-4o-mini as the default