feat(tools): add Perplexity Search tool

[james-pplx]

Summary

Adds PerplexitySearchTool — a new built-in tool that performs web search via the Perplexity Search API (POST https://api.perplexity.ai/search).

The tool follows the existing search-tool pattern in this repo (mirrors DiscoveryEngineSearchTool / VertexAiSearchTool): it subclasses FunctionTool, exposes a single query argument to the model, and lets developers pin server-side options (max_results, search_recency_filter, search_domain_filter, etc.) at construction time. HTTP calls go through httpx.AsyncClient, which is already in core deps — no new dependency is added.

Every outgoing request includes an X-Pplx-Integration: google-adk/<version> header for attribution. The API key can be supplied via constructor or the PERPLEXITY_API_KEY environment variable.

API reference: https://docs.perplexity.ai/api-reference/search-post

Files

• src/google/adk/tools/perplexity_search_tool.py — tool implementation
• src/google/adk/tools/__init__.py — lazy export entry
• tests/unittests/tools/test_perplexity_search_tool.py — 12 unit tests
• contributing/samples/perplexity_search_agent/ — example agent (mirrors google_search_agent sample)

Testing plan

• Unit tests added under tests/unittests/tools/test_perplexity_search_tool.py (12 tests, all passing locally with pytest).
• Tests use httpx.MockTransport so no live HTTP calls are made.
• Tests assert the X-Pplx-Integration attribution header, Authorization: Bearer header, request URL, request method, and the request body shape (with and without options).
• Tests cover error paths (HTTP status error, transport error) and the lazy import in google.adk.tools.
• pyink and isort (Google profile) clean.

$ pytest tests/unittests/tools/test_perplexity_search_tool.py -v
============================== 12 passed in 1.46s ==============================

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[adk-bot]

Response from ADK Triaging Agent

Hello @james-pplx, thank you for creating this PR!

To help us track and review this new feature, could you please create a GitHub issue and associate it with this PR?

Thanks!