Use configured PHP executable path in ToolExecutor subprocess

[pushpak1300]

When running Boost's MCP server via HTTP transport (Mcp::web()) behind PHP-FPM, ToolExecutor uses PHP_BINARY to spawn subprocesses — which resolves to php-fpm instead of php, breaking all tool calls. The boost.executable_paths.php config already exists and is respected elsewhere, but ToolExecutor wasn't using it.

Fixes #723

Approach

• Extracted the command normalization logic (absolute path detection, multi-token command splitting) into a shared CommandNormalizer helper so it can be reused
• Updated ToolExecutor::buildCommand() to read the configured PHP path with PHP_BINARY as fallback
• Added tests for absolute paths with spaces, multi-token wrappers like herd php, and the default fallback