feat: add LiteLLM as AI gateway provider

[RheagalFire]

Summary

Add LiteLLM as a built-in provider, giving users access to 100+ LLM providers (Anthropic, Azure, Bedrock, Vertex, Groq, Ollama, etc.) through a single OpenAI-compatible proxy.

LiteLLM proxy runs locally and translates requests to any supported provider's native format. Since it speaks OpenAI protocol, it reuses the existing OPENAI_CHAT transport with zero new transport code.

Changes:

• src/row_bot/providers/catalog.py - added litellm ProviderDefinition (base_url: http://localhost:4000/v1, transport: OPENAI_CHAT)
• src/row_bot/providers/runtime.py - added litellm to configured provider list
• tests/test_litellm_provider.py - 3 unit tests

Type of change

• Bug fix
• Feature
• Docs
• Refactor
• Test-only change
• Build / CI / release tooling

Risk area

• Agent / prompts / tools
• Designer
• Memory / knowledge graph
• Channels / external integrations
• Installers / auto-update
• UI only
• Other

Testing

• I ran python tests/test_suite.py
• I added or updated tests
• I manually tested the affected user flow
• Not applicable, docs-only change

python tests/test_suite.py: 1143 pass, 103 fail (all pre-existing failures from missing optional deps nicegui and langgraph.checkpoint.sqlite, unrelated to this change).

tests/test_litellm_provider.py::TestLiteLLMProvider::test_provider_registered PASSED
tests/test_litellm_provider.py::TestLiteLLMProvider::test_provider_definition PASSED
tests/test_litellm_provider.py::TestLiteLLMProvider::test_provider_uses_openai_compatible_transport PASSED
3 passed in 0.26s

Existing provider catalog tests: 41 passed (1 pre-existing nicegui failure).

Release notes

• User-visible change, release notes needed

Users can now select LiteLLM as a provider and point it at their LiteLLM proxy (http://localhost:4000) to access any of 100+ supported LLM providers.

Checklist

• Branch is based on latest main
• No direct secrets, API keys, local paths, or private data included
• The change is focused and does not include unrelated cleanup
• Windows/macOS behavior considered where relevant

[RheagalFire]

@siddsachar