feat(text): add OpenRouter provider via OpenResponses

[Kamilbenkirane]

Summary

• Add Provider.OPENROUTER with OPENROUTER_API_KEY auth and default base URL https://openrouter.ai/api
• Wire a minimal OpenRouterTextClient (Ollama-style OpenResponsesTextClient subclass) with optional attribution headers from OPENROUTER_HTTP_REFERER / OPENROUTER_APP_TITLE
• Use an empty model catalog so any OpenRouter slug works via explicit model= (same pattern as Ollama)

Usage

await celeste.text.generate(
    "Hello",
    model="openai/gpt-4o",
    provider=Provider.OPENROUTER,
)

Test plan

• make lint / make typecheck
• Pre-commit and pre-push hooks (668 unit tests)
• OpenRouter covered via shared OpenResponses machinery + enum/fallback patterns (no dedicated test file, matching Ollama)

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