feat: add create_proxy() factory function

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Summary

• Add create_proxy() template factory function matching Python fastmcp's API
• Add unified fastmcpp.hpp header for convenient single-include usage
• Add unit tests and documentation

Changes

1. create_proxy() factory function

Implements the create_proxy() factory function to match Python fastmcp's convenience API:

// From URL - auto-detects transport (HTTP/WS)
auto proxy = create_proxy("http://localhost:8000/mcp");

// From existing client
auto proxy = create_proxy(std::move(client), "my_proxy", "1.0.0");

Features:

• Auto-detects transport type from URL scheme (http/https → HttpTransport, ws/wss → WebSocketTransport)
• Supports creating proxy from existing Client instance
• Local tools take precedence over remote (override behavior)

2. Unified header

Adds fastmcpp.hpp main header for single-include convenience:

#include <fastmcpp.hpp>  // All common types available

3. Tests & Documentation

• 3 new unit tests for create_proxy() in tests/proxy/basic.cpp
• Proxy section added to README.md
• Compile test for unified header

Test Results

• All 76 unit tests passing ✅
• E2E interop test (Python backend → C++ proxy → Python client): 8/9 passing

Test plan

• ctest --test-dir build -C Release passes (76/76)
• Manual verification of create_proxy() with URLs and Clients
• E2E interop testing with Python fastmcp