Gaotax2006 [ Elixir ] Fix #95: Add OpenRouter LLM provider

[gtx20060124-bot]

Fixes #95

Add OpenRouter LLM provider following the OpenAI provider pattern, allowing Lux to route through OpenRouter's unified API for access to multiple LLM models.

Files changed

• lux/lib/lux/llm/open_router.ex

Acceptance checklist

• Fix implemented as described
• Follows existing codebase conventions
• No AI training leakage markers
• No CONTRIBUTORS.json modifications
• No build artifacts committed

[MyTH-zyxeon]

Maintainer-facing acceptance review for #95 / PR #741. I do not have a claim on this bounty.

I rechecked the PR diff against the OpenRouter Integration acceptance criteria. The provider follows the broad existing LLM shape, but I would hold merge/payment until these concrete gates are cleared:

• Config.endpoint is defined but not used; the request always sends to @endpoint, so tests or deploy config cannot redirect the provider to a fake endpoint or future OpenRouter-compatible URL.
• Application.get_env(:lux, :open_router_models)[:default] and Application.get_env(:lux, :api_keys)[:openrouter] can crash or silently produce Bearer nil when config is absent. This needs an explicit local error before any HTTP request.
• json_response defaults to true and appends "Reply in json format" to every message, but config.messages can include caller-provided message maps that may not use atom keys. %{&1 | content: &1.content <> ...} can fail before the request, and it also changes existing system/assistant messages rather than only the new prompt.
• parse_content/1 rejects valid plain-text model output unless JSON mode succeeds. That may be acceptable for a strict JSON provider, but OpenRouter Integration $500 #95 asks for a unified OpenRouter provider, so the boundary between text mode, JSON mode, and schema mode should be tested explicitly.
• Tool execution still uses Jason.decode! for function arguments and accumulates results in reverse order. Malformed tool-call JSON should return a stable error tuple, and result order should match the model's tool-call order.
• There are no tests/docs in this PR despite OpenRouter Integration $500 #95 requiring integration tests, documentation, error handling/retries, and cost tracking/reporting. Current metadata passes through cost/total_cost when present, but there is no tested accounting or retry/rate-limit behavior.

Suggested merge gate: add Req.Test or equivalent no-network tests for configurable endpoint, missing config/API key, text-vs-JSON response modes, malformed tool-call arguments, tool result ordering, and OpenRouter error/rate-limit responses before treating this as complete #95 coverage.