What does this PR do?

Add Megatron Bridge support for Qwen3.5 language models (Qwen3.5-LM), which are the text-only component extracted from Qwen3.5-VL. The model architecture is similar to Qwen3-Next (hybrid GDN + standard attention), but the HF checkpoint organization follows the Qwen3.5-VL convention (model.language_model.* prefix instead of model.layers.*). Additionally, the MTP mcore path is renamed from transformer_layer to mtp_model_layer to align with Megatron-Core's actual module naming.

Changelog

• Add Qwen3_5Bridge for dense Qwen3.5 language models (qwen3_5_bridge.py)
• Add Qwen3_5MoEBridge for MoE variant of Qwen3.5 language models (qwen3_5_moe_bridge.py)
• Register both bridges in models/qwen/__init__.py with model types qwen3_5_text and qwen3_5_moe_text
• Fix MTP parameter paths in qwen3_bridge.py: mtp.layers.*.transformer_layer.* → mtp.layers.*.mtp_model_layer.*
• Fix MTP parameter paths in qwen3_next_bridge.py: same rename as above (13 mappings updated)
• Add comprehensive unit tests for both bridges (test_qwen35_bridge.py)

Second commit changelog

• Rename Qwen3_5Bridge → Qwen35Bridge and Qwen3_5MoEBridge → Qwen35MoEBridge
  to follow project naming conventions.
• Merge dense and MoE text bridges into a single qwen35_bridge.py module with
  extracted helper functions and static mapping methods that are consumed by both text and VL bridges, eliminating code duplication.
• Fix incorrect HF param paths in dense LM mappings (removed spurious language_model nesting).
• Remove transpose_on_export from Qwen3.5 bridge to fix shape mismatch.
• Add functional tests for dense and MoE bridges.

Key design decisions

GitHub Actions CI

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Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? — Unit tests for both dense and MoE bridges added
• Did you add or update any necessary documentation?
• Does the PR affect components that are optional to install? — No, uses existing dependencies only

Additional Information

• Qwen3.5-LM is the standalone text model extracted from Qwen3.5-VL, sharing the same hybrid GDN+Attention architecture as Qwen3-Next but with VL-style checkpoint organization (This is because Hugging Face transformers do not accept other state dict organizations).
• Standalone Qwen3.5-LM is necessary for me and other guys. Please refer to [feature] Add qwen3.5 config + example for LLM only. #2973.
• Potential third party bug that brakes Qwen3.5-VL unit tests: (Qwen3_5MoeVisionConfig missing deepstack_visual_indexes field — silently dropped by @strict huggingface/transformers#45375).

Acknowledgements

From Engine Architecture Group 5, Engine Infrastructure Department, Xiaohongshu (RedNote).

Note to maintainers: please include the following trailers when squashing:

Co-authored-by: HuayiJin <27394199+HuayiJin@users.noreply.github.com>
Co-authored-by: whitelok <whitelok@163.com>