Previously, agent.disabled_toolsets in config.yaml only worked for CLI
mode (run_agent.py --disabled_toolsets). The gateway always passed
enabled_toolsets to AIAgent, and get_tool_definitions() ignored
disabled_toolsets when enabled_toolsets was set.
Fix: _get_platform_tools() now reads agent.disabled_toolsets from config
and excludes those toolsets from the returned set. This runs last so it
overrides everything above.
Added 3 tests covering cross-platform suppression, explicit platform
config override, and empty/missing config no-op behavior.
