fix(tools): reuse reqwest::Client in HttpTool execute() to prevent connection leaks

[vivekrajsingh04]

Description

This PR fixes a resource-management issue in HttpTool where reqwest::Client::new() was being called inside the hot path on every single tool invocation.

Because reqwest::Client holds an internal connection pool, async DNS resolver, and TLS session cache, creating a new client per call meant:

• Every HTTP request started a fresh TLS handshake.
• Keep-alive connections were never reused.
• The Tokio executor accumulated background cleanup tasks for dropped connection pools, leading to socket leaks on long-running ReAct agents querying external APIs.

Changes Made

1. Refactored HttpTool from a unit struct to hold a single client: reqwest::Client.
2. Added HttpTool::new() and Default implementations.
3. Updated execute() to use self.client, allowing the underlying connection pool to persist and be safely reused across concurrent executions.
4. Updated the builtin.rs module documentation example to reflect HttpTool::new().

Motivation (GSoC 2026)

I was auditing the scheduling and tool execution paths for my GSoC proposal (Idea 3: Unified Inference Orchestrator) to understand behavior under extended load, and noticed this un-reused pool scaling poorly for high-throughput agents.

Fixes #1542