Add T5 architecture adapter test

[RecreationalMath]

Description

Adds a unit test suite for T5ArchitectureAdapter under tests/unit/model_bridge/supported_architectures/. It needs no model downloads or real checkpoints as it uses tiny programmatic TransformerBridgeConfig objects, so it runs on CPU in seconds.

The suite (50 tests) covers:

• Adapter config defaults: RMSNorm, positional_embedding_type = "relative_positional_bias", final_rms = False, supports_fold_ln = False (fold-LN would corrupt T5 RMSNorm weights), and the gated-MLP switch driven by cfg.is_gated_act.
• Component-mapping structure: eight top-level keys covering the encoder stack (pos_embed, encoder_blocks, encoder_ln_final), the decoder stack (decoder_pos_embed, decoder_blocks, decoder_ln_final), plus shared embed and unembed.
• Top-level bridge types and HF module paths, including the two distinct relative-attention-bias paths (one per stack, on block.0.layer.0).
• Encoder block: pre-norm, single self-attention with q/k/v/o LinearBridges and requires_relative_position_bias=True, plain MLPBridge (or GatedMLPBridge in Flan-T5).
• Decoder block: self-attention + cross-attention + three layer norms; cross-attention is flagged is_cross_attention=True and the encoder self-attention must not be.
• Flan-T5 gated MLP variant: when cfg.is_gated_act=True, both encoder and decoder MLPs become GatedMLPBridge with wi_0 (gate), wi_1 (in), wo (out).
• Factory registration: dual-registered, T5ForConditionalGeneration and MT5ForConditionalGeneration both dispatch back to T5ArchitectureAdapter.
• Architecture guards: encoder must not grow a cross_attn or ln3; weight conversions stay empty.

Contributes to #1302 (T5 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