[Request] Add Serply as a web search tool option

Feature Description

Nexent's SDK ships a family of interchangeable web-search tools (TavilySearchTool, ExaSearchTool, LinkupSearchTool, ...), but there is no option backed by Google SERP results. Users whose agents need Google-flavored organic results (or who already hold a Serply subscription) currently cannot use their key with Nexent.

Proposed Solution

Add a SerplySearchTool that mirrors TavilySearchTool: an HTTP wrapper around the Serply SERP API (GET https://api.serply.io/v1/search/), subclassing the same smolagents Tool base, emitting the same observer messages and SearchResultTextMessage card format, registered via the standard three touch points (tools/__init__.py, ToolSign, tool_labels.py). The tool is optional and only active when a user supplies their own SERPLY_API_KEY.

I have a working implementation with unit tests (network mocked) and have live-tested it against the real API; I will open a PR right after this issue.

Additional Information

Serply API docs: https://serply.io/docs

Disclosure: I work with Serply. The integration follows the existing house pattern and adds no required dependency (plain httpx, already used by the SDK).

Start by comparing the existing TavilySearchTool pattern, then inspect tools/init.py, ToolSign, and tool_labels.py for the three registration points. Run the mocked unit tests mentioned in the issue; done means the optional Serply tool follows the existing message and result-card conventions when SERPLY_API_KEY is supplied.