Add Mixtral architecture adapter tests

[RecreationalMath]

Description

Adds a unit test suite for MixtralArchitectureAdapter under tests/unit/model_bridge/supported_architectures/, following the existing adapter-test pattern (modelled on the qwen3_moe and qwen2 suites). It needs no model downloads or real checkpoints, it uses tiny programmatic TransformerBridgeConfig objects, plus small synthetic tensors and a fake attention module for the behavioural tests, so it runs on CPU in seconds.

The suite (49 tests) covers:

• Adapter config defaults (RMSNorm, rotary, gated MoE MLP, final_rms=False).
• Weight conversions: QKVO weights plus Q/K/V biases, with GQA-aware head counts and the no-n_key_value_heads fallback.
• Numerical round-trips: the rearrange conversions are actually run on synthetic HF-shaped weight and bias tensors, asserting the split-head output shapes and lossless reversion.
• Component-mapping structure, bridge types, and HF module paths, including the block_sparse_moe MoE bridge with its gate router and the absence of Q/K-norm.
• Factory registration and dispatch via select_architecture_adapter.
• GQA forward hook shapes: a fake attention module wired into the bridge confirms Q surfaces n_heads while K/V surface n_key_value_heads.
• setup_component_testing rotary-embedding wiring, eager-attention forcing, and robustness on a minimal HF model.
• Architecture guards against drift.

Contributes to #1302 (Mixtral checkbox).

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[jlarson4]

Excellent work on this @RecreationalMath! Merging