feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds ExaSearchTool, a client-side BaseTool that searches the open web via the Exa AI search API. It complements the existing GoogleSearchTool and VertexAiSearchTool, which are server-side Gemini built-ins, by giving agents a model-agnostic search option that returns structured results the model can read directly.
• Exposes the practical Exa surface: search type (auto / fast / neural / hybrid / instant), numResults, category, domain and text include/exclude filters, published-date ranges, and per-result contents (highlights, text, summary) with snippet fallback.
• Authentication uses the EXA_API_KEY environment variable or an explicit apiKey on the constructor; the SDK client is created lazily so the tool can be instantiated without a key.

Usage

import {LlmAgent, ExaSearchTool} from '@google/adk';

const agent = new LlmAgent({
  name: 'researcher',
  model: 'gemini-2.5-flash',
  instruction: 'Use exa_search to find recent web sources before answering.',
  tools: [new ExaSearchTool()],
});

The model can also pass per-call options to refine the query, e.g. category: 'research paper', includeDomains: ['arxiv.org'], or a date range.

Files changed

• core/src/tools/exa_search_tool.ts — new tool, schema declaration, and response normalization with highlights → summary → text snippet fallback.
• core/src/common.ts — exports ExaSearchTool and its public types.
• core/test/tools/exa_search_tool_test.ts — vitest unit tests covering the function declaration schema, missing-key error, default + per-call argument forwarding, numResults clamping, snippet fallback, query validation, and env-var fallback.
• core/package.json / package-lock.json — add exa-js dependency.

Test plan

• npx vitest run --project unit:core core/test/tools/exa_search_tool_test.ts (8 / 8 passing)
• npx vitest run --project unit:core core/test/tools/ — all 22 tool test files (150 tests) pass, no regressions
• npx eslint core/src/tools/exa_search_tool.ts core/test/tools/exa_search_tool_test.ts — clean
• npx prettier --check on the new files — clean
• npm run build — succeeds

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