Multi-provider search with Exa → Tavily → Serper → DuckDuckGo fallback chain

Summary

This PR implements a multi-provider search system with automatic fallback for DeepResearch. It provides a 4-provider fallback chain optimized for quality and reliability.

Fallback Chain

1. Exa.ai (primary) - Best quality, semantic/neural search ($10 free credits)
2. Tavily (2nd) - Purpose-built for RAG/LLMs (1,000 free requests/month)
3. Serper.dev (3rd) - Google SERP results (2,500 free queries)
4. DuckDuckGo (final) - Free forever, always available

The system automatically falls back to the next provider when:

• API key is not configured
• Rate limit is hit (429)
• API errors occur (401, 402, 432, 433)
• Credits/quota exhausted

Why This Approach?

Scenario	Old (Single Provider)	New (Fallback Chain)
Exa credits run out	Search fails	Falls back to Tavily
Tavily quota exceeded	Search fails	Falls back to Serper
Serper rate limited	Search fails	Falls back to DuckDuckGo
No API keys configured	Cannot use	DuckDuckGo works
All providers fail	Crash	Clear error message

Provider Comparison

Feature	Exa.ai	Tavily	Serper.dev	DuckDuckGo
Search Type	Neural/semantic	RAG-optimized	Google SERP	Keyword
Quality	Excellent	Excellent	Good	Acceptable
API Key Required	Yes	Yes	Yes	No
Free Tier	$10 credits	1,000/month	2,500 queries	Unlimited
Best For	Semantic search	LLM apps	Google results	Fallback

Changes

• inference/tool_search.py: Complete rewrite with 4-provider fallback
  • search_exa(), search_tavily(), search_serper(), search_duckduckgo()
  • sanitize_query() for input validation
  • format_results() for consistent output
  • Proper error handling for each provider's specific error codes
  • multi_provider_search() orchestrates the fallback chain
• .env.example: Updated with all provider API keys

Configuration

# .env - configure one or more (DuckDuckGo needs no key)
EXA_API_KEY=your_exa_key        # Best quality (recommended)
TAVILY_API_KEY=your_tavily_key  # RAG-optimized (recommended)
SERPER_KEY_ID=your_serper_key   # Google results
# DuckDuckGo is always available as final fallback

Error Handling

Provider	Error Code	Meaning	Action
Exa	402	Payment required	Fall back to Tavily
Tavily	432	Plan limit exceeded	Fall back to Serper
Tavily	433	Pay-as-you-go limit	Fall back to Serper
All	429	Rate limited	Fall back to next
All	401	Invalid API key	Fall back to next

Testing

Tested all four paths:

• Exa primary works with type: "auto" search
• Tavily fallback works with Bearer token auth
• Serper fallback works when Exa/Tavily disabled
• DuckDuckGo fallback works when all APIs disabled
• Error messages show which providers failed and why