fix(gateway): read nested multiplex profiles config

[yungchentang]

Summary

• Bridge gateway.multiplex_profiles from config.yaml into the runtime GatewayConfig load path.
• Preserve the existing precedence where top-level multiplex_profiles overrides the nested gateway value.
• Add a focused regression test for the CLI-written nested config shape.

Root Cause

load_gateway_config() normalizes config.yaml into gw_data before calling GatewayConfig.from_dict(). The dataclass parser already knows how to read gateway.multiplex_profiles, but the loader never forwarded the nested gateway value, so configs written by hermes config set gateway.multiplex_profiles true were effectively ignored.

Fixes #52562

Tests

• python -m pytest tests/gateway/test_config.py -q
• python -m py_compile gateway/config.py tests/gateway/test_config.py
• git diff --check

[alt-glitch]

This was generated by AI during triage.

Duplicate of #51372 (earliest open canonical fix for #52562). Identical mechanism and file (gateway/config.py nested-section fallback for gateway.multiplex_profiles, mirroring max_concurrent_sessions). Prior identical twins #52596 and #52684 were already closed as duplicates of #51372. Note #51372 is a subset of the broader open #50094, which also fixes Bug-B (secret-scope isolation).

[yungchentang]

Confirmed duplicate after maintainer triage. #51372 is the earlier canonical PR for the nested gateway multiplex_profiles config path and is already approved, so I am closing this PR to avoid duplicate review work.