fix(gateway): load nested gateway.platforms config

[Que0x]

Summary

Fix gateway config loading so platform settings defined under gateway.platforms.<name> are honored at runtime.

Previously, load_gateway_config() only merged the top-level platforms: block, while CLI messaging and documentation directed users to configure bundled gateway platforms under gateway.platforms.<name>. As a result, fields like enabled, extra, and home_channel could be silently ignored when written to the documented path.

This change adds support for nested gateway.platforms loading while preserving existing precedence: top-level platforms: still overrides gateway.platforms: when both are present.

Why

This aligns runtime behavior with the documented configuration path and avoids a silent misconfiguration footgun in gateway setup.

Testing

• tests/gateway/test_config.py → 50 passed in 2.84s
• tests/test_gateway_streaming_nested_config.py → 3 passed in 0.29s

[teknium1]

Merged via #35329 (#35329). Your commit was cherry-picked onto current main with authorship preserved (0bfe19b). Combined with #34557 (same subsystem). Thanks!