config: stop writing config onto the published ServerArgs at three sites (#33334)

Each of these wrote a value after resolution so a later reader would find it on
the instance. None of them needed the instance: one write was redundant, and the
two that carry a value the resolved-config readback reports move to
get_context().override, which the readback overlays.

- The SM100 GDN prefill default was written onto ServerArgs and read back one
  line later by initialize_linear_attn_config. It is now the return value of
  flashinfer_gdn_prefill_default, threaded into initialize_linear_attn_config
  (an explicit --linear-attn-prefill-backend still wins) and recorded with
  get_context().override so /server_info reports the backend in effect.
- The XGrammar fallback recorded grammar_backend="none" on the instance. No code
  reads the field after the factory reads it once, but get_internal_state
  reports the whole resolved config, so the fallback now lands there instead:
  the readback tells the truth and the seed keeps the requested backend.
- UnifiedRadixCache.init_hicache re-applied the direct-IO layout fixup that
  __post_init__ already applies: init_hicache only runs when hierarchical cache
  is on, which is exactly when _handle_hicache normalizes page_first to
  page_first_direct (pinned by test_hicache_io_backend_and_mem_layout_
  compatibility::direct_with_page_first). Three fixtures reached the fixup by
  building ServerArgs(model_path="dummy"), whose resolution is skipped, so they
  now declare the layout resolution would have produced.

Writer ratchet 34 -> 31.

test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py

@@ -530,6 +530,7 @@ class TestUnifiedRadixCacheKVEvents(CustomTestCase):
             model_path="dummy",
             page_size=self.cfg.page_size,
             hicache_io_backend="direct",
+            hicache_mem_layout="page_first_direct",
             hicache_write_policy=write_policy,
         )
         set_global_server_args_for_scheduler(server_args)
@@ -2869,6 +2870,7 @@ class UnifiedRadixCacheSuite:
             model_path="dummy",
             page_size=self.cfg.page_size,
             hicache_io_backend="direct",
+            hicache_mem_layout="page_first_direct",
             hicache_write_policy=write_policy,
             hicache_storage_backend=storage_backend,
             hicache_storage_backend_extra_config=storage_extra_config,
@@ -6231,6 +6233,7 @@ class TestUnifiedRadixPrefetchCorruption(CustomTestCase):
             model_path="dummy",
             page_size=self.cfg.page_size,
             hicache_io_backend="direct",
+            hicache_mem_layout="page_first_direct",
             hicache_write_policy=write_policy,
         )
         server_args._mamba_cache_chunk_size = max(FLA_CHUNK_SIZE, self.cfg.page_size)