feat: add Avian as an LLM provider

[avianion]

Summary

• Add Avian as a supported LLM supplier in quivr-core
• Avian provides an OpenAI-compatible API (chat completions, streaming, function calling) at https://api.avian.io/v1
• Uses ChatOpenAI from langchain-openai since the API is fully OpenAI-compatible — no new dependencies required

Models

Model	Context	Max Output	Input $/1M	Output $/1M
deepseek/deepseek-v3.2	164K	65K	$0.26	$0.38
moonshotai/kimi-k2.5	131K	8K	$0.45	$2.20
z-ai/glm-5	131K	16K	$0.30	$2.55
minimax/minimax-m2.5	1M	1M	$0.30	$1.10

Changes

• Add AVIAN to DefaultModelSuppliers enum
• Add Avian model configurations with context/output token limits to LLMModelConfig
• Add AVIAN supplier handling in LLMEndpoint.from_config() (uses ChatOpenAI with default base URL https://api.avian.io/v1)
• Add unit test for Avian endpoint configuration

Usage

from quivr_core.rag.entities.config import DefaultModelSuppliers, LLMEndpointConfig
from quivr_core.llm import LLMEndpoint

config = LLMEndpointConfig(
    supplier=DefaultModelSuppliers.AVIAN,
    model="deepseek/deepseek-v3.2",
    llm_api_key="your-avian-api-key",  # or set AVIAN_API_KEY env var
)
llm = LLMEndpoint.from_config(config)

Test plan

• Unit test added: test_llm_endpoint_avian verifies correct ChatOpenAI instantiation, base URL, and token limits

cc @StanGirard @chloedia