Fix Mamba2ForCausalLM weight tying (#43207)
* Fix Mamba2ForCausalLM weight tying
Add _tied_weights_keys mapping to enable proper weight tying when
tie_word_embeddings=True. This is the standard pattern used by
MambaForCausalLM, GPT2, LLaMA, and other models.
Fixes #43206
* Enable weight tying in Mamba2ModelTester for regression testing
* Add explicit regression test for Mamba2 weight tying
Replace ModelTester default with explicit test per reviewer feedback.
