fix(gateway): respect explicit platform disables

[ArktosIO]

Summary

• Respect explicit platforms.<name>.enabled=false settings during plugin platform auto-enable.
• Preserve the explicit enabled marker for all platforms, not just Slack.
• Add a regression test covering Discord remaining disabled when its dependency check passes.

Test plan

• ./venv/bin/python -m pytest tests/gateway/test_gateway_plugin_platform_disable.py -q -o 'addopts='
• ./venv/bin/python -m pytest tests/test_gateway_streaming_nested_config.py tests/gateway/test_gateway_plugin_platform_disable.py -q -o 'addopts='

[alt-glitch]

Duplicate of #31596 (and #31665). All three PRs fix the same bug: _apply_env_overrides() in gateway/config.py ignores explicit enabled: false for plugin platforms. #31596 was first and takes a cleaner approach (checks enabled is False directly without requiring an _enabled_explicit marker).