feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds Exa (api.exa.ai/search) as a web-search provider, fitting into the existing Tavily / Z.AI / Ollama / DuckDuckGo fallback chain in performWebSearchForTask.
• New key is stored in Keychain (KeychainService.get/setExaAPIKey) and exposed in Settings next to the existing Tavily field. When configured, Exa is preferred over Tavily; DuckDuckGo remains the keyless fallback.
• Exa is requested with type: auto and contents: { text, highlights }. A typed Decodable response model is decoded, and a exaSnippet helper cascades through highlights → summary → text so the formatter handles whatever combination Exa returns for a given page.

Why Exa

Exa's auto search type chooses between neural and keyword retrieval per query, which complements the existing keyword-style providers and is useful for the more open-ended research queries the agent often makes. The new field is opt-in and does not change behavior for users who only have a Tavily key set.

Usage

Open Settings → "Web Search" and paste an Exa key:

exa-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Once set, calls through performWebSearchForTask (e.g. the web_search / search tool) hit Exa first.

// performWebSearchForTask fallback chain:
// 1. Ollama Web Search (if provider == .ollama and key set)
// 2. Z.AI search-prime (if provider in [.zAI, .bigModel] and key set)
// 3. Exa /search (if Exa key set)              ← new
// 4. Tavily /search (if Tavily key passed in)
// 5. DuckDuckGo HTML scrape (always-on fallback)

Files changed

• Agent/AgentViewModel/TaskUtilities/WebSearch.swift — new performExaSearchInternal, typed response decoder, snippet helper; Exa step inserted into the fallback chain
• Agent/Services/KeychainService.swift — get/setExaAPIKey
• Agent/AgentViewModel/Core/AgentViewModel.swift — exaAPIKey property bound to Keychain
• Agent/Views/Settings/SettingsView.swift — Exa API key field in the Web Search section
• Agent.xcodeproj/project.pbxproj — register ExaSearchTests.swift in the AgentTests target
• AgentTests/ExaSearchTests.swift — unit tests for response parsing and snippet fallback
• README.md — tool table updated to mention Exa/Tavily/DuckDuckGo

Test plan

• swiftc -parse passes on every modified Swift file
• ExaSearchTests covers: standard response, empty results, malformed JSON, missing optional fields, snippet cascade (highlights → summary → text), whitespace-only highlight skipping, empty-key guard
• xcodebuild -scheme "Agent!" build (requires full Xcode; my local env only has Command Line Tools, so I couldn't run it — happy to iterate if CI flags anything)
• xcodebuild -scheme "Agent!" test runs the new ExaSearchTests suite (same caveat)
• Manual smoke test: enter an Exa key in Settings, run a web_search and confirm Exa results render

Notes

• No new dependencies — the implementation uses URLSession + JSONDecoder, matching how the other providers are written.
• All existing providers (Tavily, Z.AI, Ollama, DuckDuckGo) are untouched.