Add Perplexity as optional premium web search provider

[kovtcharov]

Summary

Add Perplexity API as an optional premium web search backend alongside the existing DuckDuckGo search (shipped in PR #495).

Context

• gaia6 has agents/tools/web_search.py (~498 lines) using Perplexity's sonar model
• gaia7 PR Enhance ChatAgent with file navigation, web browsing, scratchpad tools, and write security guardrails #495 ships DuckDuckGo search via browser_tools.py (free, no API key)
• Perplexity provides higher quality search results but requires an API key ($)

Design

• DuckDuckGo remains the default (free, no config needed)
• If PERPLEXITY_API_KEY is set, agent uses Perplexity for search
• User can toggle in settings or via config

Source

Port from gaia6/src/gaia/agents/tools/web_search.py

[kovtcharov]

Status: Partially Implemented

What exists:

• PerplexityService class in src/gaia/mcp/external_services.py (lines 466-517)
• Chat Agent registers search_web tool (src/gaia/agents/chat/agent.py lines 1680-1705)
• Uses Perplexity sonar model when PERPLEXITY_API_KEY environment variable is set
• Falls back to DuckDuckGo search (from PR Enhance ChatAgent with file navigation, web browsing, scratchpad tools, and write security guardrails #495, not merged) when no API key

What's still needed:

• Merge the DuckDuckGo search from PR Enhance ChatAgent with file navigation, web browsing, scratchpad tools, and write security guardrails #495 so there's a free fallback
• User-facing toggle in settings (not just env var)
• Documentation in docs/guides/ or docs/reference/
• Test coverage for the Perplexity integration path

Note: PR #495 includes browser_tools.py with DuckDuckGo search_web + fetch_page. Once that merges, the two-tier search (free DDG + premium Perplexity) will be complete. This issue can likely close after PR #495 merges + docs are added.