[Refactor] Rename NSA → DSA: user-facing aliases, file/class/import rename (#25821)

Co-authored-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

test/registered/quant/test_deepseek_v32_fp4_mtp_4gpu.py

@@ -37,7 +37,7 @@ class TestDeepseekV32FP4DPSpecV2(CustomTestCase):
             "4",
             "--enable-dp-attention",
             "--attention-backend",
-            "nsa",
+            "dsa",
             "--moe-runner-backend",
             "flashinfer_trtllm",
             "--quantization",
@@ -127,7 +127,7 @@ class TestDeepseekV32FP4TPSpecV2(CustomTestCase):
             "--tp",
             "4",
             "--attention-backend",
-            "nsa",
+            "dsa",
             "--moe-runner-backend",
             "flashinfer_trtllm",
             "--quantization",