Add Mixtral architecture adapter tests (#1329)

* Add Mixtral architecture adapter tests

Download-free unit suite for MixtralArchitectureAdapter (49 tests, no
real checkpoints), following the existing adapter-test pattern. Covers config
defaults, QKVO and Q/K/V bias conversions with GQA head counts, component
mapping (including the MoE bridge), factory dispatch, numerical conversion
round-trips, GQA forward hook shapes, setup_component_testing, and
architecture guards.

* Minor docstring change

Author: RecreationalMath