model: add Hy3 (hy_v3) support with MTP speculative decoding (#25395)

* model: add Hy3 (hy_v3) architecture support

Adds Tencent Hunyuan 3 (HF architecture HYV3ForCausalLM, GGUF arch
hy_v3): a MoE decoder stack with per-head Q/K RMSNorm, a sigmoid
router with expert selection bias, an always-active ungated shared
expert, and leading dense block(s) (first_k_dense_replace).

The base implementation is ported from charlie12345's fork
(https://github.com/charlie12345/ROCmFPX, src/models/hyv3.cpp),
adapted to current mainline APIs (hparams.n_layer(), build_qkv,
build_moe_ffn with fused gate_up + scale tensors, output_s).

Note: blk.N.exp_probs_b is stored without a .bias suffix for
compatibility with existing hy_v3 GGUFs produced by that fork.

Co-Authored-By: charlie12345 <charlie12345@users.noreply.github.com>
Co-authored-by: Piotr Wilkin <ilintar@gmail.com>
Assisted-by: Claude Fable 5

tests/test-chat-auto-parser.cpp

@@ -1944,6 +1944,9 @@ static void test_role_markers_all_templates(testing & t) {
         // MiniMax M2: ]~b]{user|ai}
         { "MiniMax-M2.jinja",                                "]~b]user",               "]~b]ai"                     },
 
+        // HunYuan V3: <｜hy_User:opensource｜> / <｜hy_Assistant:opensource｜>
+        { "tencent-Hy3.jinja",                               "<｜hy_User:opensource｜>", "<｜hy_Assistant:opensource｜>" },
+
         // Nemotron Nano v2: <SPECIAL_11>{User|Assistant}; assistant marker
         // is followed by a prefilled <think> block that gets included.
         { "NVIDIA-Nemotron-Nano-v2.jinja",                   "<SPECIAL_11>User",       "<SPECIAL_11>Assistant" },