feat(inception): add Inception Mercury 2 LLM plugin

[joycech333]

Adds livekit-plugins-inception with support for Inception's Mercury 2, a low latency diffusion reasoning model.

• Extends the OpenAI-compatible base
• reasoning_effort: controls depth of reasoning ("instant", "low", "medium", "high"); "instant" is Inception-specific
• realtime: reduces wait time for first diffusion block (TTFT)

Example

from livekit.plugins.inception import LLM                                                                                                                                         
                                                                                                                                                                                  
agent = Agent(                                                                                                                                                                    
    llm=LLM(                                                                                                                                                                      
        model="mercury-2",                                                                                                                                                        
        reasoning_effort="low",                                                                                                                                                   
        realtime=True,                                                                                                                                                            
    ),                                                                                                                                                                            
    # ...                                                                                                                                                                         
)

Reads INCEPTION_API_KEY from env, or pass api_key= explicitly.

[CLAassistant]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.