feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds an ExaSearch built-in tool that queries Exa, an AI-powered search engine that understands natural-language queries and returns high-quality web results.
• Exposes Exa's key features: search types (auto, neural, fast, deep-lite, deep, deep-reasoning, instant), content modes (highlights, full text, summary), category filtering, include/exclude domain filters, published-date ranges, and user location.
• Uses EXA_API_KEY for authentication (loaded lazily on each call) and sets x-exa-integration: open-agent-sdk-go so Exa can attribute traffic to this SDK.
• Registered in tools.DefaultRegistry() alongside the existing WebSearch placeholder; fits the same types.Tool interface as every other built-in tool.
• Pure net/http implementation, no new Go module dependencies.

Usage

package main

import (
    "context"
    "fmt"

    "github.com/codeany-ai/open-agent-sdk-go/tools"
    "github.com/codeany-ai/open-agent-sdk-go/types"
)

func main() {
    tool := tools.NewExaSearchTool()
    res, _ := tool.Call(context.Background(), map[string]interface{}{
        "query":              "latest research on retrieval-augmented generation",
        "num_results":        float64(5),
        "category":           "research paper",
        "include_highlights": true,
        "include_summary":    true,
    }, &types.ToolUseContext{})
    fmt.Println(res.Content[0].Text)
}

Set EXA_API_KEY before running. Get a key at https://exa.ai/.

Files Changed

• tools/exasearch.go — new ExaSearchTool implementing the types.Tool interface.
• tools/exasearch_test.go — unit tests covering request shape, response parsing, snippet fallback logic, API error handling, missing-key state, and registry registration.
• tools/registry.go — register NewExaSearchTool() in GetAllBaseTools().
• README.md — document EXA_API_KEY and add ExaSearch to the built-in tool list.

Test Plan

• go build ./... passes
• go vet ./... passes
• go test ./... passes (full suite green, including pre-existing tests)
• go test ./tools/ -run Exa -v passes (6 new tests, all subtests green)
• gofmt -l clean on all new/modified files
• Registered tool is discoverable via DefaultRegistry().Get("ExaSearch")
• End-to-end smoke test against the real Exa API (manual, requires a valid EXA_API_KEY)